348 
PALAEONTOLOGY 'OF NEW-YORK. 
ral resemblance to some of the more gibbous forms of the Lower Helderbero- 
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. 
Rliyiichonella (gtenocisma) eximia. 
PLATE LY. 
Atrypa eximia : Ham,, Report on the Fourth Geol. District of Hew-York. Illustration 66,. fig-4. 
Shell ovate : valves subequally convex in young shells; the dorsal valve 
becoming gibbous in older shells. 
Ventral valve 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 H. ($.) prolifica of 
the Hamilton group ; but they are more finely plicated, and attain the size of the 
larger specimens of that species before the sinus becomes much developed. 
Geological formation and locality. This species occurs in the Chemung group, 
at Ithaca ; and is mainly confined to a single stratum, in which it is abundant. 
