322 PALEONTOLOGY OF ILLINOIS. 



acter of a mesial fold, opposite a corresponding sinus of the 

 other valve. 



Socket plates of the dorsal valve much as in Orthis, being 

 quite prominent and very diverging, with a linear, longitu- 

 dinal, mesial ridge between them. Dental lamina? of the ven- 

 tral valve, thin, prominent, very closely approximate at their 

 connection with the bottom of the valve, and with a similar 

 mesial septum, extending as three nearly parallel plates to the 

 middle of the valve. (Muscular and visceral impressions 

 unknown.) Shell structure distinctly punctate. 



The typical species of this group (Sjririfer liemiplicatus, Hall), has the gene- 

 ral external appearance, when well developed, of a subglobose Orthis, of the 

 section usually described as resupinate, excepting that it is provided with a few 

 very large plications, one of which forms a mesial fold on the dors 1 valve. 

 Internally, however, we observe a marked difference in the nature and arrange- 

 ment of the dental laminae, which at their connection with the bottom of the 

 valve, are closely approximate, and with a prominent mesial septum, extended 

 nearly parallel to each other, far forward. The close approximation of these 

 plates, indicates a different arrangement of the cardinal and other muscles, 

 from that of Orthis. In the latter genus, slight ridges usually extend forward 

 from the dental laminse, but so widely separated as to leave a large depression 

 between, for the attachment of the muscles. In the type under consideration, 

 the plates that represent these ridges in Orthis, are so close together that it is 

 difficult to understand how any muscles could have been attached to the bottom 

 of the valve between them. This will be better understood when we state that 

 these plates are rarely more than the tenth of an inch apart at their connection 

 with the bottom of the valve, and in some examples, where the shell is as much 

 as nine-tenths of an inch in diameter, the space between these laminae is no 

 where more than 0.06 of an inch across, and at least one-fourth of this narrow 

 space is occupied by the mesial septum. 



Fig. 36. 



I b c 



Syntriclasma kemiplicata. 

 The above cut, a, shows the dental laminse (I, I), and mesial septum (m), as seen in a section across 

 the beak of the ventral valve. Figure b, represents the lamince (i), as seen in a side view; also the 

 Hoeket plates (s) in the same way. Fig. (c) is an outline posterior view, of a smaller specimen, show- 

 ing the small area and foramen. 



