150 GEOGNOSY AND GEOLOGY. 
The hinged bivalves belong to the Acephala or Conchifera; the single 
unilocular shells to the Gasteropoda. The chambered shells are referable to 
the Cephalopoda, or cuttle fish order. 
The Acephalous Mollusca have a soft mucous body inclosed by a mantle 
which secretes on both sides a calcareous shell inclosing the animal. The 
shells are connected at the back by a hinge, more or less toothed. The 
hinges exhibit differences sufficient to furnish excellent distinctive characters. 
The shells are united by muscles, by means of which the animal can shut 
them at pleasure. The Acephala are divided into Monomyaria and 
Dimyaria, as the shell has one or two closing muscles. 
The Monomyaria occur in but slight development in the transition slate, 
the Ostracee being entirely wanting, while traces of the Pectinide are 
exceedingly rare. The Aviculacee are of more decided occurrence; of 
these Avicula lineata (pl. 37, fig. 15) is the most abundant. The Avicule 
have an oblique shell, with an acute process of the hinge, which carries a 
small tooth. 
The Dimyaria are found in great variety, and are especially represented 
by the Cardiaceez. These have thick, equal valves, with irregular cardinal 
teeth, and strong muscular impressions connected with the more or less 
spherical external form. Cardium lyelli (pl. 37, fig. 16), C. pectunculoides 
(fig. 17), and C. vilmarense ( fig. 18). Cypricardi@ belonging to the same 
family, differ in having oblong inequilateral valves. They have two to three 
principal cardinal teeth, the preceding genus having four. PJ. 37 fig. 19¢, 
represents Cypricardia impressa. Among Acephala the Brachiopoda are 
particularly abundant; they here attain their greatest development, 
decreasing more and more in number and variety in the more recent 
formations. They are bivalve, but recognisable by the inequality of the 
two valves, one being much larger than the-other. The genus Pentamerus, 
to which belongs P. knightii (pl. 37, fig. 19°, and pl. 42, fig. 62), has 
strongly curved beaks, and in the interior five longitudinal chambers, two 
in one valve and three in the other, formed by projecting longitudinal 
plates. 
The Strygocephala have an undulating hinge margin, over which the 
beak of the larger valve projects. A more or less regular triangular space, 
the hinge space, is thus formed, which is pierced by a triangular perforation, 
contracting with age, and becoming at last completely closed. PJ. 37, fig. 20, 
represents Strygocephalus burtini from before, and fig. 21 from the side. 
Leptena is an allied form, with the hinge margin straight, the beaks very 
close, and the cardinal area very small and without perforation; Leptena 
lata (sarcinulata, pl. 37, fig. 22). The species of the genus Orthis have in 
general the same structure as Leptena; they are distinguished by the 
presence of a perforation in the cardinal area; Orthis lepis (fig. 23). The 
Spirifers have a straight or curved hinge margin, cardinal area large, with 
a large triangular aperture and bent beaks. The species Spirifer radiatus 
( fig. 24°) and S. speciosus ( fig. 25), are very characteristic of the transition 
strata. The very distinct Terebratule are found in great abundance in 
nearly all fossiliferous strata. The beak of the larger valve is provided 
580 
