460 



MOLLUSCA. 



mented with small pits. This genus seems to be exclusively 

 Silurian. 



In the genus Siphonotreta (fig. 315) 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 communi- 

 cates 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 pre- 

 served. All the Siphonotretce at present known belong to 

 the Silurian period. 



In the genus Acrotreta (fig. 316, E-G), lastly, the dorsal 

 valve is nearly flat, and the ventral is conical and patelli- 



Fig. 315.Siphonotreta 

 verrucosa. Silurian. 



Fig. 316. A, Trematis filosa Lower Silurian ; B, Cast of Obolus Davidsoni Upper Silurian ; 

 c, Interior of the dorsal valve of the same ; D, Lingulella Davisii Upper Cambrian ; E, Ven- 

 tral valve of Acrotreta Nicholsoni, of the natural size and enlarged Lower Silurian ; F, Side 

 view of the ventral valve of the same, enlarged ; G, Interior of the dorsal valve of the same 

 enlarged. (A is after Billings, and the other figures after Davidson.) 



form, the surface being concentrically 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 Lower Silurian. 



