LAMELLIBEANCHIATA. 



GONIOPHORA CRASSA. (N. Sp.) 

 Plate 2, figs. 3, 3a, 36, & 3c. 



Shell mytiloid or nearly semi-ovate in lateral outline, transversely 

 elongated, length about twice the maximum height, narrow in front 

 and widening behind, highest a little behind the middle. Valves oblique- 

 ly sub-oarinated or strongly angulated along their centre, the angula- 

 tion extending in a curved line from the beaks to the posterior end of 

 the base, — very convex and obliquely compressed both above and below 

 the median angle, so that the outline of a transverse section through 

 both when closed would be distinctly rhomboidal. Thickness through 

 the valves, as measured on the median angle, somewhat exceeding 

 their maximum height. Dorsal margin ascending gradually in nearly 

 a straight line from the anterior terminal beaks to the upper portion of 

 the commencement of the posterior end : posterior margin broadly and 

 obliquely rounded or obliquely subtruncated, descending rather abruptly 

 and forming a somewhat angular junction with the basal line below, 

 but rounding evenly to the dorsal margin above. Ventral (or basal) 

 margin nearly straight or slightly concave from the posterior end to a 

 little in advance of the middle, then narrowing rather gradually 

 upwards towards the beaks : umbones narrow, curved, carinated, over- 

 hanging the anterior end and exteuding downwards to the basal mar- 

 gin : beaks hooked, curved inwards and downwards, and margined 

 beneath by a distinct groove. 



Surface marked with numerous, closely disposed, fine raised strias of 

 growth, also by a few distant and much coarser concentric sulcations. 

 Test very thick, especially in the umbonal region. 



Anterior muscular impression subcircular and deeply excavated: 

 posterior muscular impression more elongated, not excavated, obscurely 

 defined, except above, where it is margined by a narrow and slightly 

 raised ridge. Hinge of the left valve apparently furnished with a lon- 

 gitudinally elongated, raised tooth-like process, which runs nearly par- 

 allel to the upper margin of the anterior adductor impression, (which it 

 partly bounds above), and nearly parallel also to the upper and outer 

 edge of the hinge line, from which latter it is separated by a deep 

 groove, which widens gradually behind. 



Umbonal cavity (in one specimen at least) strongly concamerated, 

 its cavity being divided off into a number of (at least eight or nine) 

 exceedingly narrow chambers, by thin, successive and concentric la- 

 minsB of shell. 



