18 nuclliDj*. 



anterior indented ray not very conspicuous ; ribs scarcely so broad 

 as their intervals ; front extremity only slightly narrowed ; rostrum 

 much recurved, its tip above the middle ; ventral margin remark- 

 ably arched in front, and much rising behind. — Var. media {Nuc. 

 cimeata, Hanley, Rec. Bivalves, pi. 19, f. 57, not description). 

 Beaked-subovate, ventricose ; the indented ray very manifest : the 

 lower sulci narrower than the ribs ; rostrum not recurved, central. 

 — Far. ? jji/rifonnis. Beaked-oblong, much nnrrowed in front, 

 ventricose ; the indented ray distinct ; upper costal interstices as 

 broad as the ribs, lower ones less so ; rostrum not recurved, sub- 

 central. — Under a very powerful lens some minute radiating lines 

 may be discerned on the umbonal ridge (the projection of the ribs 

 u])on which is a sahent characteristic), and sometimes, too, on the 

 outer portion of the front ventral maruin. Were it not for the 

 intermediate form, the produced variety ? (for which, if a species, 

 I would suggest the name pt/riforniis, but have only as yet seen a 

 single specimen) could not possibly have been identified with the 

 more typical (or rather the first dehneated) form, the pecuharities 

 of which seem only developed in mature examples. 



30. L. BELLULA, A. Adams, f. 74. — T. subinfequilateralis, tu- 

 mida, oblongo-acuta, postice longior et acute rostrata aliquari- 

 tumque recurvata, cute pertenui cinereainduta, undiquelanielloso- 

 costellala. Margo dorsahs posticus subretusus, subdeclivis ; ven- 

 tralis in medio valde arcuatus, postice rectiusculus et conspicue 

 acclivis. Area angulo circumscripta, longitudinaliter lamelloso- 

 corrugata. Lunula parva, inconspicua.- — Peaked-oblong, swollen 

 at the umboes, much compressed behind, white under a very thin 

 not much polished cinereous epidermis, without anterior indented 

 ray, ridged throughout (dorsal areas excepted) "itli moderately 

 distant concentric lamellar costellre, which are flattened in front, 

 converge posteriorly, and surmount the upper edge of the incon- 

 spicuous umbonal ridge. Extremity of the somewhat tapering 

 and rather shorter anterior side rounded : posterior side rapidly 

 and sharply beaked; its tip angular, slightly upturned, and rather 

 above the middle. Front dorsal slope moderate, convex : hinder 

 dorsal slope rather slight, somewhat retuse. Ventral margin greatly 

 arched in the middle, rising convexly in front, peculiarly and 

 subrectilinearly ascending behind : posterior refusion, if any, 

 slight. Escutcheon angulately defined, lamellately wrinkled length- 

 ways. Lunule small, rather inconspicuous, scarcely impressed, 

 concentrically wrinkled. 



31. Tj. fhagilis, Chemnit:,i. ^t^ft^ — T. sequilateralis, subtumida, 



