BRACHIOPODA. 149 



spiniferous Cranias never evince more than a very slight tendency to surface 

 modification from attachment. 



The shell-structure in this genus is essentially calcareous and composed of 

 two layers, the inner of which is the thicker, the outer being thin, dark colored, 

 epidermal. It is strongly punctated by vertical tubules, the openings of which 



Fig. 68. Vertical section of tlie sliell of tlie upper valve Fio. 69. Horizontal sectionjust below the 



of Crania anomala (after King), showing the arbores- upper .surface. After King. 



cence of a rubule on approaching tlie outer surface. 



on the internal surface may, under good preservation, be seen with the naked 

 eye. It has been shown by Carpenter,* KiNGf and Joubin,:j: that these tubules 

 are widest at their inner extremities, and as they reach the epidermal layer in 

 the upper valve they ramify and become aborescent, making an irregular plexal 

 opening on the surface. From Joubin's observations this aborescent character 

 does not appear to exist in the attached valve. 



In respect to muscular anatomy there is little reason to doubt the closer re- 

 lation of Crania to Discina and Discinisca than to any other of the inarticulates. 

 The four large impressions in each genus occupy correlative positions, and 

 though Mr. Davidson terms the posterior pair in Crania {Pseudocranm) divaricata, 

 divaricators, and those in Discinisca, adductors, we can find no authority for 

 ascribing different functions to the muscles they represent. But in the entire 

 absence, in any stage of the development of the shell, of pedicle or foramen, 

 and in the calcareous nature of the shell-substance. Crania stands far apart 

 from all other members of the class ; and Joubin has observed that it is the 

 only known brachiopod which has the anal opening exactly in the median line. 



There is no evidence that Crania has been a member of faunas older than 

 the Silurian, and no indubitable proof of its appearance earlier than the Tren- 



*0n the Intimate Structure of the Shells of the Brachiopods : Davidson's Introduction to British Fossil 

 Brachiopoda, chap. ii. 



t On the Histology of the Test of the Class Palliobranchiata : Trans. Royal Irish Academy, vol. xxiv. 



t Recherches sur TAnatoniie des Bivachiopodes Inarticnl(5s : Archives de Zoologie Experimentale, t. iv. 

 1886. 



