652 MOLLUSCOIDEA. 



The genus is characteristic of the Cambrian rocks of Canada. Schmidtia, 

 of the Silurian and Devonian rocks, has a minute oval shell, the ventral 

 valve of which is convex, with a pointed beak below which an "area" is 

 developed ; and the genus Leptobolus of the Ordovician rocks of North 

 America is closely allied to this, but differs in its internal characters. In 

 the Cambrian genus Acrothele, the shell is nearly circular, and the ven- 

 tral valve is somewhat conical, the beak being sub-central and perforated 

 by a foramen, while the beak of the dorsal valve is marginal. 



In the genus Sifthonotreta (fig. 497) the shell is oval, inequivalve, with 

 unarticulated valves. The beak of the ventral valve is perforated by a 

 foramen which opens on its back, and communicates with the interior by 

 a cylindrical tube. The surface of the shell is 

 covered with concentric lines of growth, and fur- 

 nished with numerous delicate tubular spines, 

 which, however, are rarely preserved. All the 

 SiphonotretcB at present known belong to the 

 Ordovician and Silurian periods. 



In the genus Acroireta (fig. 495, E — G) the 



Fig. wj.—SiphoJiotrcta j -, ,° in,. j A. . i ■ 



verrucosa. Ordovician. dorsal valve is nearly flat, and the ventral is 

 conical and patelliform, the surface being con- 

 centrically striated. The beak of the ventral valve is perforated by a 

 round foramen, from which a pedicle-groove extends to the posterior 

 margin, and is bounded laterally on either side by a small false area. 

 The genus is wholly Ordovician. 



In the neighbourhood of Acrotreta, Dr Fischer, with some doubt, 

 places the Silurian genus Eichwaldia, which has been commonly referred 

 to the articulate family of the Rhynchonellidce, but in which hinge-teeth 

 and sockets appear to be absent. The shell in this curious genus is sub- 

 trigonal or oval, and the ventral valve has its apex perforated by a 

 peduncular foramen, while the dorsal valve is provided with a prominent 

 median septum. The structure of the shell is very peculiar, the most 

 conspicuous feature connected with this being that the greater part of 

 the surface is covered with a special alveolated layer, the hexagonal 

 pits of which are arranged in decussating oblique lines. 



Lastly, the Carboniferous rocks of India have yielded certain curious 

 types which have been placed in this family, and have been referred to 

 the genera Neobohts, Lakhmina, and Schizoftholis. In all these genera 

 the shell is of small size, and either suborbicular or trapezoidal in shape. 



Family 3. Discinid^e. — In this family the shell is corneo-cal- 

 careous, with unequal valves, the animal being attached by a 

 muscular peduncle passing through the ventral or lower valve by 

 means of a slit in its hinder portion, or a circular foramen exca- 

 vated in its substance. The arms (fig. 499) are fleshy and spirally 

 coiled, and are not provided with calcified supports. The Discinidce 

 range from the Ordovician period to the present day. 



In the genus Disciiia (figs. 498-500) the shell is sub-orbicular, with 

 conical, nearly equal valves, of which the dorsal is the deepest, while the 

 ventral valve is flatter, and the beaks of both are sub-central. In the 

 typical forms of Discing the ventral valve has an elongated marginal 

 opening, which is prolonged on the surface into an anterior groove tra- 

 versing the shell obliquely and terminating internally in a second groove 

 which runs in an opposite direction to the first. In the centre of the 



