498 PHILOSOPHY OF ZOOLOGY. 



we do not consider the mere form of the shell as furnishing 

 characters sufficiently obvious and precise to warrant such 

 distribution. 



"55. TEIIEBIIATULA. Valves unequal, the peduncle pass- 

 ing through an aperture in the largest valve. 



The arms are shorter than those of the Lingula, and are 

 said to be forked. They are supported within by numer- 

 ous arcuated plates. 



M. LAMAHCK divides the recent kinds into two sections. 



1. Shell smooth, or destitute of longitudinal ribs. The 

 T. cranium, a native of the Zetland seas, may be quoted 

 as an example. The peduncle is simple *. 



2. Shell ribbed longitudinally. The T. aurita -f-, which 

 inhabits Loch Broom, is another, though recently discover- 

 ed British example. The larger valve is broadest in the 

 middle, semicircular in front, and narrowing towards the 

 apex, in consequence of the sides being compressed or bent 

 inwards. The ribs from the beak towards the anterior 

 margin are the most distinct, rounded, and about eight in 

 number ; those towards the sides are obsolete. The under 

 valve is nearly orbicular, with the margin at the hinge 

 truncated, or rather obtusely angular, and having the sides 

 depressed, and forming small auricles, as in the genus Pec- 

 ten, but not produced. The ribs are obsoletely wrinkled 

 across, and the margin is waved by the ribs being concave 

 internally. The inner surface of both valves, especially the 

 largest, is finely punctulated. The hinge is formed by a 

 projection on each side, the proximal margin of the per- 

 foration in the large valve entering corresponding depres- 

 sions in the smaller one. The margin of the perforation 

 itself is completed by the application of the smaller valve. 



* See Plate IV. f. 4. t Plate IV. f. 5, 



