Dilemma, Leal, Jos Ẽ H., 2008

Leal, Jos Ẽ H., 2008, A remarkable new genus of carnivorous, sessile bivalves (Mollusca: Anomalodesmata: Poromyidae) with descriptions of two new species, Zootaxa 1764, pp. 1-18 : 3-4

Dilemma new genus

Diagnosis. Shell apparently nacreous internally, strongly compressed in anteroposterior direction and laterally expanded; shell compression associated with ca. 30º rotation of the largest dimension (height) in relation to the anteroposterior axis; lateral outline cardioid; height larger than width; width larger than length. Byssus very well developed (indicating attachment for life); foot vestigial, never leaving infraseptal chamber; posterior byssal retractor muscles wider in cross-section than posterior adductor muscles; anterior labial palps very well developed, cup-shaped; posterior labial palps very reduced, consisting of short projections flush to surrounding surface.

Description. Shell apparently nacreous internally, thin, equivalve (except for slight anteroposterior overlap at umbo), inequilateral, strongly compressed in anteroposterior direction, lateral outline cardioid, anteroventral surface flat or slightly concave, height larger than width, width larger than length. Umbones projecting dorsally, anterior. Byssal gap anteroventral when present.

Hinge with sub-triangular, cardinal-like tooth in each valve; tooth in left valve with cleft distal extremity and more developed than that in right.

Ligament external, sunken; inner ligamental layer apparently calcified, white. Outer ligamental layer brown, splitting and diverging anteriorly, with each half curved as a result of tangential component of shell growth; periostracum fused externally with outer ligamental layer. Lithodesma absent.

Posterior byssal retractor scar larger than posterior adductor scar, elongate. Lunule absent; escutcheon smooth, well-defined.

Animal compressed in anteroventral to posterodorsal direction. Mantle lobes thin, except in siphonal area, with three apertures (see below).

Incurrent siphonal length and siphonal opening diameter larger than equivalent dimensions in excurrent siphon. Incurrent siphon eversible, resting inside infraseptal chamber. Incurrent siphonal opening separated from byssal gape by well-developed ventral portion of perisiphonal suture.

Base of incurrent siphon surrounded by up to 15 tentacles; tentacles simple, tapered, with one large unpaired tentacle along fused mantle margin dorsal to excurrent siphon; pair of tentacles present internally, in supraseptal chamber, between posterior margin of excurrent siphon and posterior adductor muscle.

Septum thin, perforated by byssal opening and by groups of ostia arranged to roughly define three pairs of line segments deployed in septum around byssal opening.

Adductor muscles almost parallel to attachment surfaces on valves. Anterior adductor short, flattened; posterior adductor longer, also flattened. Posterior byssal retractors very well developed, attaching to locations close to posterior margin of valves. Anterior byssal retractors thinner, attaching dorsally to internal surface of entrance to umbonal cavity. (I prefer to use byssal retractor muscles instead of pedal retractor muscles.)

Byssus well-developed, prominent, circular in cross section. Vestigial foot dorsal to byssus, narrow, or consisting only of an expansion around the middle region of byssus. Position and size of vestigial foot indicates that it can never exit infraseptal chamber.

Mouth indicated by beginning of pleats that continue inside esophagus. Posterior labial palps reduced to two small projections flush with surrounding surface and united along midline. Posterior palps not extending posteroventrally on either side of vestigial foot. Anterior labial palps strongly developed, large, folded in the shape of cups over mouth, free from anterior adductor muscle, potentially expandable in posteroventral direction (toward siphonal area).

Esophagus very short, with strong internal folds, penetrating stomach anterodorsally. Stomach of Type II, compact, strongly muscular, situated in dorsal orientation in relation to mouth, between two “horns” that house digestive diverticula and ovary. Midgut relatively short, connecting to stomach in anterodorsal position, ventral to esophagus. Testes situated in anteroventral position in relation to ovary, ovary overlying the testes. Kidneys situated posterodorsally between remaining of visceral mass and posterior byssal retractor muscle. Kidneys comprising two elongate structures, one on each side of midline of posterior region.

Type Species. Dilemma frumarkernorum new species, by original designation.

Etymology. The generic name is the Greek noun dilemma , a proposition consisting of questionable alternatives. It is used in this case to denote the impasses faced by the author in the course of this work.