( 28 ) 



4. Birostritjes. Lam, 



Foss. Interior of shell un- 



Inequivalved, valves known. (1) 



elevated at their disk, un- 

 equal, diverging, almost 

 straight, like horns, one 

 enveloping the other at 

 the base. Fie. 95. 



5. Discina. Lam. Foss.(2) 



A ronnded oval, a little 

 depressed ; valves of equal 

 sue, each with a very dis- 

 tinct, central, orbicular 

 disk : disk of upper valve 

 not pierced, with a protu- 

 berance in the middle ; 

 that of the lower, very 

 white, divided by a trans- 

 versal cleft. 



6. Crania. Brug. (3) 



Inequivalved, sub-orbi- 

 cular, inferior valve 

 smooth, pierced internally 

 by three unequal oblique 

 holes, upper valve very 

 convex, with two pro- 

 jecting callosities within. 

 Fig. 91 . 



Dianchora . Sowerbj 

 Foss. 



Inequivalved, adhe- Hinge without teeth, 



rent ; the attached valve 

 with an opening instead 

 of a beak, the other valve 

 beaked and eared. (4) 

 Fig. 96. 



(1) The valves diverge like a very open V, appearing as if one proceeded from 

 the base of the other; it is always the shortest which is enveloped. 



(2) It is found on the coasts of Great-Britain. There is no trace of a hinge or 

 ligament, or any distinct muscular impression. 



(3) The Crania personata is said to be the only species living. The fossils are 

 found in Sweden ; at Meudon, Nchou. 



(4) Found in Lower Chalk and Green Sand. 



