164 Natural History Bulletin. 



oblongus too it was extremely prolific, so much so that por- 

 tions of the beds are crowded full of casts that interlock and 

 intersect with one another to such an extent as to make it 

 difficult to obtain a perfect specimen. One peculiarity of these 

 casts is that while the original shell substance has been dis- 

 solved and carried away, a fine grained material resembling 

 the dolomitic matrix has been substituted in its place, retaining 

 all the markings of the original surface and giving us practi- 

 cally perfect casts of the exterior of the shell. On some of 

 the specimens the thin layer that took the place of the original 

 shell, scales off in places and reveals the internal cast. The 

 species may be described as follows: 



Pentamerus decussatus WhiteavesA Plate XI, Figs. 1-3; 

 Plate XII, Fig. 2. 



Shell rather large, very variable both as to size and shape; 

 average forms subovate, widest in front of the middle, moder- 

 ately convex; anterior margin sinuous with the middle some- 

 what abruptly produced. 



Dorsal valve convex in the umbonal region, flattened or 

 even concave from side to side toward the front; beak pro- 

 jecting into foramen of other valve; no hinge-area. 



Ventral valve much more convex than the dorsal; most 

 convex at the umbo, but arching somewhat regularly from 

 beak to front margin; middle of valve occupied by an imper- 

 fectly defined subangular ridge; two other ridges, one on 

 each side mid- way between the median ridge and the lateral 

 margins sometimes faintly defined, sometimes obsolete; beak 

 incurved; foramen broadly triangular, its base occupied by 

 beak of dorsal valve; hinge-area imperfectly defined. 



Median septum on interior of ventral valve very short as 

 compared with P. oblongus, rarely extending much beyond 

 the umbo; V-shaped pit, moderately large; the two septa of 

 dorsal valve fairly well developed. 



I Pentamerus decussatus, (sp. nov.) Whiteaves, CauadiiDi Record of Science, 

 April, iSgi, p. 295, Plate III, Figs. 3 and 4. 



