BRACIIIOPODA. 



673 



The surface is apparently smooth, but is in reality ornamented with 

 minute, close-set circular pits disposed in radiating lines. The species 

 of Porambonites are characteristic of the Ordovician rocks, and are 

 especially abundant in deposits of this age in the Baltic provinces of 

 Russia. 



Family 7. Terebratulidce. — The shell in the Terebratulidce is 

 minutely punctate in structure, and is rounded or oval in form, the 



Fig. 540. — Interior of the ventral valve (a) and dorsal valve (b) of the recent IValdheimia 

 Jiavescens. a, Beak of the ventral valve with the foramen (/) ; t, Hinge-teeth ; /, Septum of 

 dorsal valve ; /, Brachial loop ; s, Dental sockets ; a a, Adductor-impressions ; b b, Ventral ad- 

 justers; c c, Divaricator-impressions ; b', Peduncular muscles; c', Cardinal process. (After 

 Davidson.) 



surface being smooth or striated. The beak of the ventral valve 

 (fig. 540, a) is prominent, and is perforated by a foramen, through 

 which passes the peduncle of attachment, and which is partially 

 surrounded by a deltidium of one or two pieces. The arms are 

 supported by a loop-shaped brachial process (fig. 540, /), which 



Fig. 541. — a, Terebratula quadrifida — Lias ; b, Terebratula spharoidalis — Inferior Oolite ; 

 c, Terebratula digona— Bradford Clay, Forest Marble, and Great Oolite (Jurassic). (After 

 Davidson.) 



varies in length and form in different types of the family, and which 

 springs from the hinge of the dorsal valve. As regards its geolo- 

 gical range, the family of the Terebratulidce is represented in de- 

 posits as old as the Devonian, or even the Silurian ; but it attains 

 its maximum development in the Mesozoic and Kainozoic deposits, 

 vol. 1. 2 u 



