WAVEBLY GROUP SPECIES. 291 



sometimes abruptly produced in the form of slender, long, very sharp, 

 spine-like extensions ; lateral margins converging, with more or less 

 convex outlines forward to the front, which is narrowly rounded or sub- 

 angular at the middle. Dorsal valve depressed convex in the central 

 region, and compressed near the posterior lateral angles; beak projecting 

 little beyond the hinge line, and, with the narrow area, rather strongly 

 incurved ; mesial fold narrow, rounded, and depressed, or scarcely rising 

 above the general convexity of the central and umbonal regiohs, but 

 usually becoming somewhat more elevated at the front, defined on each 

 side by a little larger and deeper furrow than those between the larger 

 costa3. Ventral valve of nearly the same general form as the other; 

 mesial sinus moderate, and narrowly extended to the beak ; area rather 

 narrow ; beak moderately prominent and incurved. Surface of each 

 valve ornamented by about thirty to thirty-six rather small radiating 

 costse, some six to eight of which usually occupy the mesial fold at the 

 front in mature specimens, and five to seven the mesial sinus, those of the 

 fold and sinus, as well as one or two of the larger ones on each side, bifur- 

 cating, while the others are generally all simple ; crossing the whole, 

 numerous fine concentric strise and an occasional stronger mark of. 

 growth may be seen on well-preserved specimens. 



Greatest breadth of^a mature specimen with lateral extremities pro- 

 duced as mucronate extensions (measuring to the points of the latter), 

 1.75 inches; do., exlcusiveof the spines, 1.20 inches; length, 0.59 inch. 



The lateral extremities of this shell seems to be always acutely angu- 

 lar, and often mucronate ; but the individual figured has them unusually 

 produced and pointed, as well as remarkably abruptly projecting from 

 the posterior lateral margins, which generally curve outward a little 

 into the bases of the spines, instead of meeting them at obtuse angles.* 

 The costse op the mesial fold usually commence at or very near the beak, 

 as two very small, obscure divisions, which soon bifurcate so as to form 

 four, the lateral two of which subdivide at some point farther forward, 

 and sometimes the middle two bifurcate toward the front so as to make 

 eight altogether. The furrow between the middle two costee of the mesial 

 fold, as well as those between the lateral margins of the fold and the 

 cost£e on each side, are usually a little larger and deeper than the others. 

 This character, and the bifurcations of the costse on the fold, and one or 

 two of the lateral ones on each, are not well shown in our figure. 



Although I believe this to be the species described by Prof Hall under 



*This angularity at the connection of the posterior lateral spines and the lateral 

 margins, however, is slightly exaggerated in figure 5. 



