THE BRACIIIOrODS. 471 



When we examine the superior surface of the beak, we 

 may observe that the perforation is completed by two small 

 triangular pieces soldered together on the mesial line, and 

 upon the margins of the beak itself. These pieces are ap- 

 parent in almost all Terebratulse, — in a rudimentary state 

 in some species, but becoming large in those which have a 

 greatly prolonged beak. From the shape and connection of 

 these pieces excellent specific characters may be derived. 

 They do not constitute of themselves the entire upper sur- 

 face of the beak ; and there are examples of brachiopod 

 adherent shells, with the beak of the dorsal valve consider- 

 ably elongated, which show no trace of them. This surface, 

 to which M. de Buch applies the term area, has much ana- 

 logy with the beak -like apex of the oysters and of the Spon- 

 dyli. In the Terebratulas we may also notice, on each side 

 oi" the pieces of the beak, a more or less extended surface 

 which corresponds to the beak of the Crania and the The- 

 cidea ; this surface is generally circumscribed externally by 

 an angle, and it is further indicated by its lines or stria? of 

 growth. 



In Lingula the valves are equal ; in Crania and Orbicula 

 the ventral patelliform valve is larger than the dorsal ; but 

 in the majority of Brachiopods the ventral valve is smaller 

 than the dorsal. Not only is it smaller, but its beak is never 

 perforated, and it is often bent to bury itself underneath the 

 cardinal hinge of the dorsal valve. This is the case in the 

 greater number of Terebratula and of Productus, and in all 

 the Thecideae. Sometimes it is straight, as in Lingula; 

 sometimes recurved as in some Terebratulae ; and in Crania 

 and Orbicula the apex of the ventral valve is more or less 

 elevated and subcentral. 



But the most admirable part pj„. qq 



connected with the hinge of the 

 shells of the Brachiopods is the 

 testaceous frame-work called the 

 " appareil apophysaire" by Des- A 

 hayes, the " internal skeleton" by 

 Owen, and intended to support 

 the fringed arms of the animal, 

 and to keep the valves open, or 

 even to assist in opening them 

 (Fig. 90), for in the Brachiopods 

 there is no cartilage at the hinge for this purpose. There is 

 no trace of this apophysis to be found in the genera Lingula 

 and Orbicula ; but rudiments of it appear in some Crania? ; 

 and it goes on through many states until it reaches very con- 



