TEREBRATULA. 



Terebratula, Llhwyd. 



Ter. vitrea (dorsal valve). 



d. Cardinal process, b. Hinge-plate to loop. s. Sockets. A. 



Quadruple impression of the adductor. 

 n. Outer groove. 



m. Inner groove. 



Ventral valve. 



1. No prominent rostral plates, only a simple 

 thickening of the shell, at the dental projections, 

 which leave no slits in the beak of internal casts. 



2. In the interior two diverging grooves (tn) ex- 

 tend from the extremity of the beak to a little more 

 than half the length of the valve ; there are also two 

 other smaller lateral ones (») ; so that in casts four di- 

 verging ridges may at times be perceived placed at 

 nearly equal distances. 



3. The muscular impressions are close together, 

 and occupy a small space at a short distance from 

 the extremity of the beak ; they are more or less 

 indented, according to the thickness of the shell, 

 and consist of a small, central, oval scar left by the 

 adductor, on either side of which are placed the 

 larger cardinal ones, and outside of these again may 

 be seen the pedicle muscular impressions, which vary 

 in dimensions in different species according to the 

 size of the pedicle and foramen. 



Dorsal valve. 



4. In the interior a small cardinal process or boss 

 projects from under the extremity of the umbonal 

 beak(</) ; thehinge-plate is divided, and consistsof two 

 rather wide and somewhat oblique shelly plates (b) ; 

 from the prolonged extremities of these depart the 



Semxnula, M'Coy = Epithyris, King. 



T. elongata, Schlotheim (interior of dorsal valve 



with part of the ventral one). 



H. Rostral or dental plates of ventral valve. S. Sockets of 

 dorsal valve. C. b. Hinge plates. L. Loops. A. Adductor 

 impressions. 



Ventral valve. 



1. Well-defined dental or rostral plates, leaving 

 slits in the beak of casts. 



2. In the interior there exists a mesial longitudi- 

 nal ridge, extending from the extremity of the beak 

 to about two thirds of the length of the valve, and 

 two shorter sub-parallel ones, situated at a small 

 distance from the central one. 



In casts these produce slight grooves, the central 

 one being particularly evident. 



3. The muscular impressions appear to be similar 

 to those of Terebratula proper, and have left but 

 slight impressions in the interior of the shell or on 

 the casts of the specimens that have come under my 

 notice. 



Dorsal valve. 

 4. The cardinal process seems to be but slightly 

 protruded ; the hinge-plates varying somewhat in 

 detail in different species, and even individuals. There 

 exists first a testaceous ridge or plate (C), which forms 

 at the same time the inner socket walls, and two 



