RHYNCHONELLID^ OP THE HAMILTON GROUP. 341 



This beautiful and symmetrical species differs from li. ho7'sfordi in its raove 

 robust character and stronger plications. It may be compared with the H. jlexistria 

 = R. tumida of Phillips, a Devonian species of Europe. 



Geological formation and localities. From calcareous layers in the Marcellus 

 shale near Leroy, Genesee county ; also in the Hamilton shales at Geneseo and 

 York, Livingston county ; and at Eighteen-mile creek in Erie county. A specimen 

 I'eceived from Rev. E. J. Bush was obtained in the vicinity of Hamilton in Madi- 

 son county, New-York. 



Rliynclionella (Stciiucisma) congregata. 



PLATE LIV. 



Jilrypa congregaia : Cokrad, Annual Eeport on the Palaeontology of New-York, p. 55. 1841. 



Shell robust, varying from short-ovate to subglobose ; length and width 

 nearly equal or a little wider than long ; front rounded or straight in 

 the middle ; apex pointed. 



Ventral valve convex at the sides, depressed in the middle ; mesial 

 sinus often beginning at about one-third the length from the apex, 

 and becoming conspicuous towards the front : beak, in old shells, 

 closely arcuate over the apex of the opposite valve ; in young shells, 

 nearly straight or slightly incurved. 



Dorsal valve gibbous in old shells, regularly convex in young specimens : 

 mesial fold scarcely conspicuous on the upper half of the shell, some- 

 times prominent near the margin ; sides curving abruptly to the junc- 

 tion with the opposite valve. 



Surface ( in young shells ) marked by only nine or ten distinct subangu- 

 lar or rounded plications ; in older shells, by eighteen to twenty-two, 

 of which three or four occupy the mesial sinus and four or five the 

 mesial fold. In the two larger specimens observed, there are but three 

 plications in the sinus, and this is the prevailing number. Slender 

 concentric striae, sometimes a little imbricated in front, mark the sur- 

 face of the shell. 

 Casts of this species are more common than the shell ; and those of 



the ventral valve show the dental plates reaching to the bottom of the 



