14 
K. L. Prendergast. 
Diagnosis . — Shell large^ semicircular in outline, greatest width at hinge- 
line; ears large and reflexed, offset from flanks, flanks convex to steep. 
Pedicle valve sinuate, regularly curved through 270°. Visceral disc occupying 
about half curvilinear length of valve, rugae and costae regular and equally 
prominent on visceral disc, costae becoming irregular on trail, two or more 
coalescing to form irregular folds; rugae absent on trail. Small spines, 
quincuncially arranged, arising at intersection of costae and rugae, with 
larger spines, more widely spaced, on rest of shell. Row of spines between 
ears and flank and row along cardinal margin. 
Brachial valve geniculate; ornamentation as in pedicle valve. 
The size of the specimens is shown by the table : — 
Syntypes. 
Paratypes. 
Largest spec., 
Irwin R. — 
12400. 
Length of hinge-line 
59 
59 
58 44 + 
49 + 
Height 
37 
38 
33+ 33 + 
46 + 
Curvilinear length 
70 
71 
50 50 
95 
Shells too crushed to allow measurement of thickness. 
Description of the syntgpes . — Shell semicircular in outline with the 
hinge the greatest width of the shell. 
The pedicle valve is curved through an angle of 270°. The transverse 
curve of the valve is a high arch with almost vertical sides and a shallow 
median depression. A shallow sinus arises at about the middle of the 
visceral disc and continues forward^ becoming shallower and Anally disappear- 
ing on the trail thus leaving the margin of the valve entire. The ears are 
large and reflexed; each is separated from the body of the shell by a sulcus, 
and this is flanked by a fold bearing a roAV of spines (3-4). A second row 
of spines runs parallel to the hinge-line and close to it. The semireticulation 
is marked on the visceral disc, covering 35 mm. along the curvilinear length, 
the costae and rugae being equally prominent. The costae cannot be traced 
to the tip of the umbo, where they have been probably I'emoved by weather- 
ing, but traces of them can be seen on the ears. They increase in number on 
the visceral disc botJi by division and by intercalation ; no increase takes 
place on the trail and the costae lose their height, remaining scarcely elevated. 
The rugae are closely spaced near the umbo, becoming farther apart 
posteriorly. The spines on the visceral disc are small and oblique and 
arranged in approximate quincunx, each spine arising from a node at the 
intersection of a ruga and a costa. Spines do not arise from all the nodes. 
As the spines are regularly spaced and the number of costae increases an- 
teriorly, there is some irregularity, but every fourth to sixth ruga bears 
spines, these being separated by eight or more costae. The larger spines on 
the trail also tend to quincuncial ari’angement. Below each on the trail is a 
fold which replaces the two or more costae above each spine, this folding 
being independent of the costae, though two or more are absorbed on each 
fold. 
