No. 89.] 65 



GENUS RHYNCHONELLA( Fischer, 1809); 



With Observations on the R. (Jtrypa) increbescens. 



The shells of this genus have been for many years included under the Genus 

 Atrtpa of Dalm AN, and in part under Terebratula, etc. by most American and 

 many European palaeontologists; and it is only recently that the name Rhyncho- 

 nella has been fully understood in its application, and generally adopted by 

 ■writers. At the same time we have already shown that some shells, usually refer- 

 red to Rhtnchonella, possess internal characters incompatible with that genus; 

 and I have separated Trematospira, Rhtnchospira and Eatonia from among 

 those species having externally the rhynchonelloid form. The two first possess 

 internal spires, while the last has certain peculiarities separating it very clearly 

 from Rhtnchonella proper. 



Since some of the species having the external form and plicated surface of the 

 genus are proved to be distinct, we may reasonably look for others among the 

 great number of species now referred to that genus from the different geological 

 periods, as it is recognized from the Lower Silurian epoch to the present time. 



Leaving out of view in this place the structure of the animal, the following 

 description will convey an idea of the character of these shells in their fossil con- 

 dition. The animal, in its living state, is free, or attached to submarine objects by 

 a pedicle which protrudes from a foramen beneath the apex of the beak. 



Generic characters. " Shell inequi valve, variable in shape, transverse or 

 elongated, circular or trigonal : valves more or less convex, with or without a 

 mesial fold or sinus; beak entire, acute, prominent, or so much incurved as to 

 leave no free space for the passage of pedicle-muscles ; foramen variable in its 

 dimensions and form, placed under the beak, exposed or concealed, entirely or 

 pg,rtially surrounded by a deltidium, the aperture being sometimes completed 

 by a portion of the umbo of the smaller valve; deltidium in two pieces, at times 

 extending in the form of a tubular expansion, or rudimentary; surface striated 

 or plaited, rarely smooth; structure fibrous, impunctate. Valves articulating 

 by means of two teeth in the larger (ventral ) and corresponding sockets in the 

 imperforate dorsal valve; apophysary system, in the smaller or dorsal valve, 

 composed of two short flattened and grooved lamellae, separate and moderately 

 curved upwards, attached to the deeply divided hinge plate. In the socket valve 

 the quadruple impression of the adductor muscle is clearly defined, and sepa- 

 rated by a short medio-longitudinal ridge s. The pedicle scars occupy the small 

 cardinal plates, between which is the small and narrow cardinal process. In the 

 perforated valve the two strong diverging cardinal teeth are supported by dental 

 plates extending to the bottom of the valve, and at the base of these a semi- 

 circular ridge curves on each side, forming a more or less defined saucershaped 

 depression into which were affixed the shell and pedicle muscles : these last 

 leave two narrow elongated scars close to the inner base of the dental laminae; 

 the remaining and largest portion being chiefly occupied by the cardinal mus- 

 cles, which are longitudinally divided by a small raised ridge; above these again 

 is seen a small oval scar due to the adductors."* 



* Davidson's Introduction to the Study of the Brachiopoda. 



[Senate No. 89.] 9 



