364 



CRANIA DM. 



Through certain curious shells, belonging to extinct 

 genera, we can trace a very gradual passage from Tere- 

 bratula and its allies to the patelliform Brachiopods, con- 

 stituting the family of which Crania may be regarded as 

 the type. In this group the shell is either fixed to extra- 

 neous substances by the agglutination of one of its valves, 

 or by a pedicle passing through a fissure or perforation in 

 the lower valve. Most probably all the Craniada are in 

 some stage of their existence affixed by a pedicle. The 

 living members of the family all belong either to the genus 

 Orbicula or to Crania. Species of both genera have lived 

 during every epoch in the world's history, and maintained 

 a striking resemblance to each other. Those of the former 

 genus are wonderfully preserved, even in the very oldest 

 fossiliferous strata, in consequence of the horny texture of 

 their valves. At present, half a dozen Orbicula; and half 

 as many Crania, are the living representatives of the tribe, 

 which, like other families of Brachiopoda, held a much 

 more important position anciently in the population of the 

 seas, than it holds now. None of the shells have true 

 hinges or cardinal ligaments. 



Dr. Carpenter describes the microscopic structure of the 

 shell of Orbicula as closely resembling that of Lingula. 

 He states, that it is almost entirely composed of lamina; 

 of horny matter, which are perforated by minute tubuli, 



