126 



BRITISH SILURIAN BRACHIOPODA. 



Valves almost equally convex ; beak small, much incurved and truncated by a small 

 circular foramen, partly margined by a very short deltidium in two pieces. Dorsal valve 

 convex and ornamented with from ten to sixteen angular ribs, of which the two central 

 ones are much smaller or narrower than those which occupy the lateral portions of the 

 valve, and are also at a slightly lower level. Ventral valve ornamented in a similar 

 manner ; but a deepish narrow sinus, commencing at the extremity of the beak, extends 

 to the front, with one, two, or three small longitudinal ribs along the middle. The 

 surface of each valve is crossed by numerous concentric raised lines, and covered with 

 small asperities due to numerous short hollow spinules. In the interior of the dorsal 

 valve spiral lamellae, for the support of the oral arms, are attached to the hinge-plate. 

 Length 6, width 7, depth 5 lines. 



Obs. In 1848 I described, under the designations of Terebratula Salted, T. Baylei, 

 and T. Bouchardii, shells which de Verneuil, Alcide d'Orbigny, and myself, then con- 

 sidered to present diflFerences sufficiently marked to make it desirable to maintain them 

 as separate species. Mr. Salter and Dr. Lindstrom have since considered R. Salteri and 

 R. Baylei to belong to a single species. In their view my two friends are possibly 

 correct; but it will be desirable to point out the reasons which in 1848 induced both 

 de Verneuil and myself to separate them. 



R. Salteri is wider than long, and ornamented with strong prominent angular ribs, 

 and with four smaller longitudinal ribs along the middle portion of the dorsal valve. Of 

 these the two central ribs are the narrowest ; these four ribs forming a very slightly 

 raised mesial fold. In the sinus of the opposite valve there is usually a narrow central 

 rib, with another larger one on each side, while the ribs on the lateral portions of the 

 valve are much larger and more prominent. Now, in B. Baylei the shell is smaller, and 

 either as wide as long or longer than wide ; and it has but one small central rib in the 

 dorsal valve, and two corresponding ribs in the ventral. R. Bouchardii is larger than 

 the two })receding shells, and has many more and comparatively smaller ribs on the lateral 

 portions of the valves ; while in the dorsal valve there exist four very narrow central ribs, 

 and three corresponding ribs in the sinus of ventral valve, but of these the median rib is 

 by far the narrowest. These details will be better understood by a glance at the figures ; 

 but the difference which struck me most in 1848, and still does now, is that, while in 

 R. Salteri and R. BoucJiardii the extremity of the beak is truncated by a small circu- 

 lar foramen, partly margined by a deltidium, in R. Baylei, on the contrary, the beak 

 was entire as in Rhynchonclla, with a small circular foramen under its angular extremity, 

 entirely margined by a deltidium. Smaller differences we also observed in the com- 

 parative number of concentric lines, as weU as in the proportions of the spinules with 

 which the surface was covered.^ It is very possible that these differences may be only 



' Dr. Carpenter, who has at my request kindly examined the shell-texture of this species, states that 

 it has the true Terebratuline perforations, large and distinct, all over the shell. It agrees in this respect 

 with Relsia. 



