﻿RHYNCHONELLA. 



99 



half the distance, and sometimes even reach nearly to the extremity of the beak and 

 umbone. This character is well exhibited in Martin's type, and may be seen likewise in 

 figs. 9 and 16 of my plate. The specimen represented in the ' Petrificata Derbiensia' 

 exhibits five large rounded ribs on the mesial fold of the dorsal valve, and four in the 

 sinus of the ventral one, these last being indented or grooved along their centre to the distance 

 of a couple of lines from the margin ; but the shorter ribs on the lateral portions of the 

 valves are not so marked, nor do they extend to any great distance from the margin. 

 Now if we examine a large number of specimens of B. pugnus, it will be seen that in the 

 generality of young and middle-sized examples the ribs are acute, and rarely present the 

 indentation along the middle ; this last-named peculiarity is common, but not general, to a 

 smaller species, B. pleurodon, and I must hasten to observe that although by far the 

 larger number of specimens of the last-named shell appear to be well distinguished 

 from B. pugnus by the more numerous ribs which cover the entire surface of the 

 valves ; in some exceptional cases they become likewise gradually obsolete as they 

 approach the beak and umbone, and in which condition are less easily separable from 

 Martin's shell. 



The prevailing number of ribs on the mesial fold in B. pleurodon is five, but that 

 number is the exception in B. pugnus, where three, four, and six on the fold ; and two, 

 three, and five in the sinus would appear to be the usual number. The ribs are likewise 

 sharper in young specimens, or races from certain localities than from others, and in this 

 condition approach most to B. pleurodon. The surface of the valves, when perfectly 

 preserved, appear to be finely striated, as is also the case with B. acuminata and some 

 other species. 



In young specimens of the shell under description the beak is acute, and but slightly 

 incurved, and in this state the small circular foramen surrounded by its deltidium may be 

 distinctly observed, but in more aged individuals, from the beak becoming much incurved 

 over the umbone of the opposite valve, the aperture can no longer be seen. Some speci- 

 mens are also much more transverse than others, and but few appear to have attained the 

 dimensions of the larger ones represented in our plate. From B. acuminata and B. 

 reniformis, Martin's shell may be distinguished by its lateral ribs. 



The interior arrangements and impressions are those of the genus Bhynchonella. In 

 the dorsal valve the shelly processes (for the support of the spirally coiled brachial appen- 

 dages) consist of two short, flattened, grooved lamellae, separate and moderately curved 

 upwards (PI. XXII, fig. 13), being prolongations of a deeply divided hinge-plate. Four 

 muscular impressions produced by the adductor or occlusor muscle are visible on the 

 bottom of the valve. In the ventral valve the hinge-teeth are supported by dental plates ; 

 scars formed by the adductor or occlusor; cardinal or devaricator; and ventral adjustor 

 muscles may be distinctly recognised, as well as the vascular markings. Professor M'Coy 

 appears to have been the first naturalist who observed in this species the slender curved 

 lamellae above described ; for, at p. 156 of his ' Synopsis,' we find a woodcut representation 



