Borniola pasca, Raines & Huber, 2012

Raines, Bret & Huber, Markus, 2012, 3217, Zootaxa 3217, pp. 1-106 : 55-57

Borniola pasca sp. nov.

Figures 28 A–F

Type Material. Holotype: LACM 3154, 1 single LV, 3.2 mm. Paratypes 1–3: LACM 3155, 3 single valves up to 2.2 mm from Tahai, 27°07’20” S, 109°26’30” W, 30–80 m. Paratypes 4–10: LACM 3156, 8 single valves up to 3 mm from the type locality. Paratypes 11–13: LACM 3157, 2 articulated specimens and 1 single valve, up to 3.4 mm, Motu Iti, 65 m. Paratypes 14–16: Coll. MHU, 1 articulated specimen and 2 single valves from Motu Iti, 65 m.

Type Locality. Dredged at 50–100 m, in sand, La Perouse Bay, Easter Island, 27°04’26” S, 109°16’50” W .

Description. A small, opaque white, shape-variable Borniola with a well defined disjunct dentition and the typical radial sculpture. Shell small, below 3.5 mm, variable in shape from rounded ovate to subquadrate, equivalve; no lunule; inequilateral; umbones prominent, pointed, opisthogyrate, posteriorly placed. Prodissoconch, P1 smooth, D-shaped, weakly defined, ca. 175 µm length by 142 µm height, P2 round, commarginally lined, ca. 356 µm length by 306 µm height. Adult valves rather compressed; non-gaping. Sculpture of dense, somewhat irregular, commarginal growth lines, crossed by dense, diverging radial striae. Hinge rather solid, without cardinals. RV with two strong, divergent, elongate laterals separated by a small depression; LV nearly edentate with a weakly expressed extended anterodorsal lamellar ridge parallel to the margin and a shorter, slightly more curbed lamellar ridge, parallel to the posterior margin. Weak hinge plate present. Pallial line continuous. Ligament internal, subumbonal. Margins smooth.

Comparative diagnosis. The hinge configuration somewhat approaches the genus Kurtiella Gofas & Salas, 2008 . They (2008: 122) interpreted the spread teeth in the RV, but also the marginal ridges in the LV as laterals. As such species of Kurtiella have no cardinal teeth, but two marked laterals in the RV and much weaker dentition in the LV. However, the remote biogeography and distinct shape exclude it from this group. Moreover a weak hinge plate is present in B. pasca and the subumbonal pit in the RV is much less pronounced than in species of Kurtiella .

From the Hawaiian Islands nothing similar is known, but numerous Borniola species from New Zealand are comparable (e.g. Powell, 1979: fig. 97). Notably, Borniola powelli Crozier, 1966 , shares traits in hinge and shape with B. pasca .

The genus Borniola was erected by Iredale (1924: 207) for the Sydney Bornia lepida ( Hedley, 1906) . Iredale gave no distinguishing remarks other than “very interesting species”. Hedley (1906: 543) illustrated the denticulate B. lepida hinge with two divergent teeth in one valve, but did not comment on the dentition in the other, nor did Chavan in Moore (1969: N523). Though reversing the left and right valves, Laseron (1956: 16) captured the Borniola species well, illustrating the type and other specimens, and noting the other hinge as edentate. As such, the hinge configuration within the genera Kurtiella and Borniola is comparable, although with a weak hinge plate still present in Borniola . In addition the genus Borniola is characterised by its radial sculpture. The shape is compressed, typically subequilateral, but in most species inequilateral. Borniola species are small in size, rarely surpassing 10 mm.

Remarks. Due to a simple, non-reflected and non-tentaculate mantle, a single inner demibranch in most species and the hinge morphology of Borniola neatly sits in between Kurtiella and Mysella . Today, Borniola is placed within the montacutinids. Per Bouchet et al. (2010), the family Borniolidae Iredale & McMichael, 1962 , is treated as invalid.

Distribution. Borniola pasca is rather common from several locations around Easter Island, in fine sand from 30–100 m, also in subtidal caves in Motu Tantarra, EI, 35–40 m — E1.

Etymology. The name reflects its rather common occurrence at Isla de Pascua.