- T09 - 



tweiitj; to thirty lov/, roixi^ded ribs, separate"! by intersoaees of 

 v-.j- +;-j^g .,c;.i;ie \7idth; on tu" osterior half of the shell the rihs 

 small, hnt increasing in width toward the center of the shell 

 where there is a single prominent rih; on the anterior h:^if the 

 ribs fe!7er wider an?. "— •~^'-- -nd decreasing in size anterior- 



ly, bixt near the marp-in of the Irnule again becoraing larger; 

 on the dorsal marp-in of the right valve one or tv/o more or less 

 irregular ribs, rnn.ch more ororninent than those i. my lately suc- 

 ceeding, present; ribs ornamented vath short, spines, most promi- 

 nent - lar^e meciisn rib and on the posterior ribs often 

 teJcing the form of small irregular nodules; ii.'';tT;:3;juoes and lun- 

 ule irregularly pitted; concentric sculpture of irregularly dis- 

 posed, slia-T^-tly imbricating lamellae near the m.argins, the lam- 

 ellae continuing across the limule; inner ventral mar ""in of the 

 chell slightly undulate "" by the anterior ribs. 



Dimensions .- Lat. , 52.5 mm.; alt., 5S mm.; semidiam, , ,"^1 nim.. 

 Puemarks .- ihe large, massive shel?.s of this species are 

 a very conspicuous element in the fa.u.na, bein<^- equalle''. in sise 

 only by some of the larf^er snccimens of Area halidonata . b^it in 

 '.veness xmsur'^assed. x'he prominent irregularly spinose rib, 

 or ribs, adjacent to the dorsal In is a constant feature on 



right valves, but is not -oresen'. ';he other valve. Som.e dist- 



ance from the dorsal m.argin there is often present a shallow, 

 but well-defined, groove. The portion of the shell betv/een this 



