INVERTEBRATES. 331 



Aviculopecten Occident alis, Shumard? 



PI. 27, figs. 4, 5, 5a. 



Pecten occidentalis, Shumard, 1855. Missouri Geological Report, part ii, p. 207. 



Peeten Cleavelandicus, Swallow, 1858, Trans. St. Louis Academy Sciences, i, p. 184. 



Aviculopecten ? Meek and Hayden, 1865. Palaeontology of the Upper Missouri, 



p. 50, pi. ii, fig. 10. 



Shell exclusive of the ears, broad subovate or flabelliform ; 

 length from beak to ventral margin, slightly greater than the 

 antero-posterior diameter ; umbonal margins converging to the 4 ■ -^ fx*c~lplC*>*. 

 beaks at an angle of 80° to 85°; lateral borders rounding into tP**- 4 ^t//7 ""** 

 the regularly rounded pallial margin; hinge line nearly or * ~JT" !2SuL 3 



quite equaling the greatest breadth of the valves. Left valve ^^^^w^*-* ,/<«*♦ 

 distinctly more convex than the other ; ears subequal, the ante- ,ww- CtL- l*^*» a«! 

 rior one being slightly shorter, more convex and more obtuse **~ ;r &— pm9h**+t 

 than the other, as well as more distinctly defined by the um- ^f** 

 bonal angle — sinus separating each from the lateral margins l^j-, » ~\. £ - ,'. - 

 rather shallow, that on the posterior side being more broadly nvtt * ^^^ ^ &&*** 

 rounded than the other ; surface of each ear ornamented by "fL* twX i f-.nV o^^» 

 eight to ten or twelve unequal, small radiating costse, which TtL. fU.rt-<t* v>* v» ** 

 are crossed by numerous very fine, closely arranged concentric /, *v" w »T JJlS t*r2 

 striae ; body of the valve similarly marked, about twelve of the *^" 

 larger costse reaching the beak, and the smaller intermediate t • . !j^ j*^v\^b 

 ones dying out at various distances between the beak and the «L>' jULL*',** *-• r»j 



ones dying out at various distances between the beak and the .A-}' MU**",** *«• f»c< 

 pallial margin ; one on the posterior umbonal slope larger and ™ A - * fc "*^y**-*~ *■ ^j*- 

 more prominent than any of the others. Right valve nearly 

 flat and much more faintly ribbed than the other, with which iSflL ^jh' ^* Jf ^ y 

 it agrees in form, excepting that the byssal sinus under the &t*',i*}^ 

 anterior ear is much deeper and more sharply defined. Cardi- 

 nal area of moderate breadth, finely striated and inclined over 

 towards the left valve.* Diameter from beaks to ventral mar- 



* In good casts of the area of this species, we have observed unmistakable evidences 

 of a very shallow, flattened, trigonal cartilage pit. It is unlike that of Pecten, how- 

 ever, being quite broad, distinctly triangular and very oblique, more as we see in 



