472 TERMINOLOGY OF THE BRACHIOPODS. 



siderable complexity in the Terebratula, Procluctus, and The- 

 cidea. The value of these modifications to the systematist 

 is not great, for he cannot avail himself of them to separate 

 the shells into natural families or even genera. In both Pro- 

 ductus and Terebratula, there are species which have the 

 apparatus well developed and spirally twisted, and others in 

 which it is reduced into simple lamina?, more or less project- 

 ing. Rudimentary in certain Crania, the apparatus continues 

 to be very simple in the greater number of the Terebratulae, 

 but it complexes itself in others. Two little ossicles spring 

 and diverge from the hinge margin, and bifurcate towards 

 their points ; the lower prong projecting horizontally, meets 

 that of the opposite side, and thus forms a primary arch, as 

 is observable in a certain number of species; the other prong 

 shoots more or less into the centre of the valve, and it is 

 either simple or curved back upon itself to form a branch 

 parallel to and above the first. Approaching the beak, the 

 extremity of this branch or prong bends back horizontally, 

 meets that of the opposite side and coalesces with it, forming 

 in this way a large arch placed above the first.* In the fos- 

 sil genus Spirifer of Sowerby, the osseous prong, which con- 

 stitutes the large arch, is twisted into a pyramidal spire or 

 cone. The figure (Fig. 90) will give you a good idea of the 

 " appareil apophysaire," in its fullest development. 



On the inner surface of the valves of different genera 

 of Brachiopods, near the middle, there may be observed a 

 notable thickening or elevation. This appears under the 

 form of an obtuse longitudinal rib in the thin valves of the 

 Lingula ; but in some Terebratulae it becomes more deve- 

 loped. There is one in the ventral valve, and two in the 

 dorsal valve, which are separate and divergent. In many 

 species these crests are short and obtuse, not reaching be- 

 yond the centre of the valves; but in other species they reach 

 the inferior margin, attain considerable prominence, and di- 

 vide the shell into three equal parts. In the Calceola there 

 is a single median crest with rugae on each side ; and in The- 

 cidea this crest is very prominent, and has on each side a 

 large apophyseal apparatus analogous to that of the Spirifer. 



* Sowerby calls the appareil apophysaire, the " shelly processes," which 

 lie says, " are sometimes short, simple, and recurved, but sometimes of con- 

 siderable length, branched and variously bent, and generally anastomising : 

 these generally commence on each side of the hinge, but sometimes near the 

 centre of the shells, and they are sometimes united to the shells at other 

 points." — " Terebratula" in Gen. Ret: and Foss. Shells. 



