CRANIA, MONOMY ARIA. 69 - 
bones ; slightly truncated or trilobate at the base : muscular 
impressions situate towards the centre of the valves; ex- 
ternal surface covered with a glossy, thick epidermis ; 
hinge destitute of teeth ; shell suspended by a cylindrical 
fleshy tendinous pedicle, attached to the umbones. 
Lingula Mytilloides. Plate VII. fig. 8. Found in the 
Carboniferous Limestone, at Wolsingham, county of Dur- 
ham. 
When the valves of this shell are closed, they are compressed, and have much 
the appearance of a duck’s bill. They are attached to each other by the in- 
ternal muscles, They vary considerably in their internal and external con- 
formations ; one valve having in its interior an elongated projection between 
the two muscular impressions, lying in a longitudinal direction ; the other 
valve is destitute of this, and also wants the testaceous matter at the points of 
the valves. 
The recent species of Lingule are few, and inhabit the Mollucca Islands, 
Fossil species of Lingule are found in the sandy indurated Marl at Bagnor ; 
in the Carboniferous Limestone of Durham, and in the shale of the Vale of 
Todmorden. 
Genus 1. — PYCNODONTA. — Fischer. 
Generic Character. — Shell inequivalve, transverse, equi- 
lateral ; hinge arcuated, extending considerably, and ending 
in subauriform lobes; numerous small, close-set, curved, 
lamelliform teeth, with a transverse, ovate pit at the centre 
for the reception of the ligament ; beaks small and but 
slightly produced. 
Pycnodonta radiata. Plate XI. fig. 16. 
Genus III, —CRANIA. — Retzius. 
Generic Character. — Shell inequivalve, suborbicular, 
mostly equilateral, slightly irregular ; upper valve patelli- 
form, very convex, interiorly provided with two projecting 
callosities, its umbo placed rather behind the centre ; lower 
valve adherent, nearly flat, pierced on its end or surface 
with three unequal and oblique holes; each valve with four 
muscular impressions ; two of those in the upper valve, are 
situate near the posterior margin, the other nearer the 
centre, but always close to each other ; in the lower valve 
two are almost marginal, and remote, but the other two are 
nearly central, and so close together that they seem united ; 
with usually a small projection between them ; destitute of 
a hinge. Animal without a byssus. 
Crania striata. Plate VIII. figs. 6, 7. 
The lower valve in this genus is exceedingly variable in thickness; in some 
