Chthamalus robustus sp. nov.

Fig. 2 (1a–8b)

Diagnosis: Wall plates thick with parietes markedly costate externally; scutum broader than long with apical angle rounded, adductor muscle pit rounded close to occludent margin, adductor ridge absent, lateral depressor muscle pit crossed by crests; tergum very narrow, without spur, with prominent articular ridge.

Etymology: Morphological, alluding to the robustness of the plates ( robustus = robust in Latin).

Occurrence: Ferrière-Larçon, Pauvrelay (Paulmy commune), Indre-et-Loire, France; faluns of Middle Miocene age.

Material: Holotype MNHN A25746: right scutum. Paratypes: MNHN A25747 broken right scutum showing crests of the depressor muscle pit; MNHN A25748– A25749 right terga; MNHN A25750 rostrum; MNHN A25752 carina; MNHN A25751 lateral; MNHN A25753 rostro-lateral. Non-figured paratype: MNHN A25754 rostro-lateral.

In addition to the type series, other numerous unregistered plates from the type localities are at my disposal.

Measurements: Holotype MNHN A25746: occludent margin 1.9 mm, basal margin 1.7 mm, articular margin 1.3 mm. Paratype: MNHN A25747: articular margin 1.7 mm. Paratype: MNHN A25748: articular margin 1.4 mm. Paratype: MNHN A25749: carinal margin 1.25 mm. Paratype: MNHN A25750: height of the middle of the plate 1.2 mm. Paratype: MNHN A25752: height of the middle of the plate 1.0 mm. Paratype: MNHN A25751: height of the middle of the plate 1.6 mm. Paratype: MNHN A25753: height of the middle of the plate 2.8 mm. Non figured paratype: MNHN A25754: height of the middle of the plate 4.7 mm.

Description: Wall plates thick, with external surface of parietes markedly costate, generally white to light or dark grey.

Scutum short, wide, articular margin as long as basal margin, apex rounded, externally broad, well spaced growth ridges distinct on uneroded portions; tergal margin with prominent articular ridge with rounded outline; articular furrow rather deep; adductor ridge absent; adductor muscle pit rounded, deep, with distinct margins; depressor muscle pit crossed by crests.

Tergum very narrow, externally broad, growth ridges not very close; articular ridge prominent, crests for depressor muscle faint; no spur.

Affinities: Chthamalus robustus sp. nov. differs from C. ligusticus in having thick wall plates, which are not divided by broad radial sulci, and from C. stellatus in having thick wall and opercular plates, a scutum with an adductor muscle pit close to the occludent margin and a lateral depressor muscle pit crossed by crests. The opercular plates of C. robustus sp. nov. have affinities with the larger ones of the extant species Chthamalus anisopoma Pilsbry, 1916 . However, the opercular plates of C. robustus sp. nov. can be distinguished from those of C. anisopoma: the scuta of C. robustus sp. nov. have no radial sulcus, which divides off the tergal third, and the terga have no spur.