166 



TWENTIETH REPORT ON THE STATE CABINET. 



reasons, has been separated. The shell is externally strongly ribbed, and 

 the texture is finely punctate throughout its substance. The ventral area is 

 well defined, narrow and linear. The fissure or foramen is very large and 

 wide, and is excavated above the area line, coming quite up to the beak, 

 and sometimes even including the apex which is worn away or absorbed. 



The teeth, which are a little separated from the margins of the foramen 

 and not continuations from it, are strong and thickened below, while they 

 are deeply crenulated on the summit and exterior margins. There is a nar- 

 row low median ridge in the cavity of the valve ; and the divaricator mus- 

 cular impressions have not been satisfactorily observed. 



The dorsal valve has a narrow area, and a wide and strong cardinal pro- 

 cess which nearly or quite fills the foramen of the opposite valve. This 

 process is often simple exteriorly, above the limit of the smooth or striated 

 pseudo-deltidium which covers it near the hinge-line; but just within the 

 valve it is broadly grooved in the middle, usually with two small deep pits 

 just within the external smooth callosity, and on each side there is a groove 

 and accessory lobe, frequently not conspicuous. The divisions made by the 

 median groove diverge and terminate below in obtuse processes which have 

 some similarity with the bases of crural processes in Orthis, but have 

 more analogy with the Terebratulidse. These processes are sometimes clearly 

 broken at their termination, but are often smooth as if the roughened sur- 

 face had been cicatrized during the life of the animal. Below these forks of 

 the process there is a narrow median crest or septum which reaches beyond 

 the middle of the valve, and sometimes nearly to the front. From the limbs 

 of the thickened divergent processes there proceed slender crura, which, at 

 first bending slightly outwards, send off a short spur into the ventral cavity 

 and are thence directed forwards, and gently curving, join the median crest, 

 to which they are attached, forming a loop of peculiar character. The occlu- 

 sor muscular impressions have rarely been seen with any degree of distinct- 

 ness; but the depressions just at the termination of the crural processes, 

 and on each side of the median ridge, are striated ; and this striation often 

 extends in a wide flabelliform expansion, probably due to vascular impres- 

 sions. Towards the margin, the interior of both valves is strongly pustulose. 



The accompanying wood-cuts illustrate the parts referred to above. 



Fig. 1. 



Interior of the dorsal valve. 



j. Cardinal process. 

 h. Crenulated teeth-sockets. 

 c. Crural processes. 

 I. Loop. 

 s. Septum. 



