No. 89.] 



6Y 



The accompanying figures illustrate the ordinary forms of the species. 

 8 7 



Fig. 6. A young specimen, front view. 7. An older specimen. 8. Front view of fig. 2. 



10 



11 



Fig. 9, 10 & 11 are front, profile and cardinal views of a full -grown individual which has become 

 very ventricose. 



Its wide geographical distribution and considerable vertical range make it an 

 interesting species. In its external features it presents no variation or diflference of 

 importance from other Rhynchonellee ; but the interior structure of specimens 

 recently examined shows some peculiarities, which, if constant, are incompatible 

 with the characters of that genus as determined from existing and some well- 

 marked fossil species. 



This species, like some others of the genus, becomes extremely gibbous or ven- 

 tricose with age, and the apex of the ventral valve is closely incurved over the 

 beak of the opposite valve. Nor is this all; for the beak is perforate, and in many 

 specimens we are able to discover a distinct foramen in the substance of the shell : 

 indeed sometimes this foramen is above or exterior to the apex of the beak, but 

 it is rarely possible to distinguish the continuity of the substance of the shell 

 between this foramen and the beak of the opposite valve. Externally, therefore, 

 this feature might not be considered incompatible with Rhynchojstella, where 

 the base of the foramen is often formed by the beak of the dorsal valve; and it 

 might be supposed that as the shell increased, and the incurvation became too 

 great to permit the protrusion of the pedicle at the ordinary foramen, the notch 

 in the beak might be deepened until it would reach beyond the apex. Sometimes, 

 however, this foramen is seen to be surrounded by the substance of the shell; 

 thus becoming a simple perforation, vrithout the appearance of deltidial plates. 



The real condition and relations of this foramen I have recently been able to 

 determine satisfactorily, from an examination of some separated valves and im- 

 perfect specimens collected by Mr. Woolson, of Iron Ridge, Wisconsin, from the 

 green shales beneath the iron ore. The interior of the dorsal valve has the usual 

 aspect of this valve of other Rhynchonellae, except that in the centre of the apo- 

 physary process, at the base of the crura, there is a narrow central process which 

 is more distinct than usual. In the ventral valve there are two strong teeth, which 

 fit into deep sockets in the opposite valve ; and above these, the triangular space 

 is partially or entirely occupied hy a concave solid area; beneath which, extending 



