a48 PALEONTOLOGY OF NEW-YORK. 



ral rcseml)lance to some of the more gibbous forms of the Lower Helderberg 

 group, this species is so distinct as to render its identification easy. 



Geological formation and localities. This species occurs in the Tully limestone, 

 at Tully and at Tinker's falls in Onondaga county ; at Ovid in Seneca county ; at 

 Benton and Penn-Yan in Yates county. New- York. 



Rli}TichonelIa ( IStenocisma ) exiiiiia. 



PLATE LV. 

 Jtrypa tximia : Hall, Report on the Fourth Geol. District of New-York. Illustration 66, flg. 4. 



Shell ovate : valves subequally convex in young shells ; the dorsal valve 

 becoming gibbous in older shells. 



Ventral vdlve moderately convex on the umbo, becoming somewhat 

 gibbous in older shells ; at about one-third the length from the beak, 

 becoming depressed in a wide defined sinus, which bears from four to 

 six plications, and usually not abruptly incurved in front : beak 

 incurved and not closely appressed. 



Dorsal valve regularly and moderately convex in young shells, more 

 convex, or a little gibbous in the middle, in old shells ; mesial fold 

 becoming visible about the middle or above the middle of the shell, 

 and often quite prominent near the front. 



Surface marked by about twenty-six, to thirty or more, angular plica- 

 tions, of which from four to six are depressed in the sinus and a cor- 

 responding number elevated on the fold. The plications become 

 obsolete on the cardinal slopes, and gradually increase in size to the 

 centre. The shell is usually exfoliated, so that the finer surface 

 markings are not preserved. 

 The partial casts of the dorsal valve show the presence of a median 



septum reaching half the length of the shell. The apex of the ventral 



valve appears to have been perforate in the young shells. 



In this species the young have the form and proportions of Ji. (S.) jrroUfca of 



the Hamilton gi-oup ; but they are more finely plicated, and attain the size of the 



larger specimens of that species before the sinus becomes much developed. 



Geological fonnution and locality. This species occurs in the Chemung group, 



at Ithaca ; and is mainly confined to a single stratum, in which it is abundant. 



