66 [ Senate 



The accompanying figures illustrate the generic character as above. 



Fig. 2. 



Fig. 3. 



Fig. 1. 



Fig. 1. The dorsal valve of R. nigricans, a living species, showing the spiral appendages (or 



labial arms) : a, the adductor muscles; i, the intestine. 

 Fig. 2. R. psittacea, ventral valve : /, foramen; d, deltidium; t, teeth; a, adductor; j>, 



pedicle muscles ; r, cardinal muscles ; o, ovarian spaces. 

 Fig. 3. Dorsal valve : c, crural processes ; t', sockets; a. adductor; s, septum. 



Figures 4 and 5 are illustrations of the ordinary form and appearane of Rhyn- 

 ckonella psittacea, as it occurs in the modern tertiary deposits of the St. Lawrence 

 valley. 



Fig. 4. 



Fig. 5. 



Species of this genus are common in the rocks of New-York, from the Chazy 

 limestone to the Chemung group inclusive. Among the most common forms in the 

 Lower Silurian rocks are the R. altilis of the Chazy limestone, R. increbescens 

 and R. dentatus* of the Trenton limestone and the Hudson-river group. In the 

 Niagara group of New-York the R. cuneatus is usually the most conspicuous form, 

 while other species are of frequent occurrence. In the Lower Helderberg group the 

 more common forms are of the type of R. wilsoni, while R. concinna and others 

 represent the type of R, dentatus. The globose forms, with rounded or flattened 

 plications, attain their greatest development in the Lower Helderberg group, while 

 the higher rocks are usually marked by the presence of the more nearly triangular 

 forms with angular plications. 



RHYNCHONELLA INCREBESCENS. 



This species is very abundant and widely distributed : it occurs in the Trenton 

 limestone of New-York and the Western States. In the higher beds of the calca- 

 reous shales of the Hudson-river group in Kentucky, Ohio, Indiana, Illinois, 

 Michigan and Wisconsin, it everywhere abounds, often attaining a large size and 

 very gibbous and ventricose proportions .' 



* These were described under the generic name of Atrypa, in the first volume of the Palaeon- 

 tology of New-York, and subsequently ( Regents' Report for 1859) placed under the Genus 



RHYNCHONELLA. 



