BRITISH SILURIAN BRACHIOPODA. 197 



126. Strophomena arenacea, Salter. Dav.j Sil. Mon., PI. XLII, figs. 6 — 8 ; Sil. Sup., 



PI. XVI, figs. 2—5. 



Mrs. R. Gray has collected this species plentifully as internal casts, and impressions of 

 the exterior, from the Upper Llandovery at Camregan Wood, in Ayrshire ; also from the 

 Middle Llandovery at Woodland Point, in the same district. Specimens which I cannot 

 distinguish from it have also been procured from the Caradoc at Duntercleugh Burn, 

 Dumfriesshire. 



127. Strophomena deltoidea, Conrad. Dav., Sil. Mon., PI. XLII, figs. 1 — 5; Sil. 



Sup., PI. XV, figs. 16—22. 



This species has been fully described and illustrated at p. 293, and in PI. XLII of my 

 * Silurian Monograph.' Since then it has been suggested to me that our English 

 specimens might perhaps be specifically distinct from Conrad's type. I therefore made 

 further researches into the matter. 



Mr. Whitfield, who is a very accurate and able worker, writes me that there are two 

 shells in the Trenton rocks at Trenton Palls, Conrad's locality for his type (' American 

 Report Geol. Survey of New York,' 1839), both of which would answer his description, 

 which is as follows : — " 8. deltoidea. Shell deltoid, with numerous radiating striae and 

 concentric rugose undulations, obsolete on the inferior half of the valves ; inferior valve 

 slightly concave above, gibbous, abruptly rounded and flattened at the base ; striae small 

 and crowded, one or ttoo lines in the middle of the valve larger and more 'prominent than 

 the others I angles of the cardinal line slightly prominent. Length 1 inch. Locality, 

 Trenton Palls." The sentence which Mr. Whitfield has underlined is the only one 

 which gives any chance of distinguishing the shells by this description. The striae of the 

 form which Mr. Whitfield has generally identified with Prof. Saffbrd's S. incrassata, Hall, 

 comparing it with Tennessee specimens, are regularly alternate, larger and smaller, in 

 fascicules, as in 8. alternata, Conrad, only they are stronger in the middle section of the 

 shell than on the sides. This leaves the middle one a little the strongest. Mr. Whitfield 

 continues to say : " On the one which has the area on the concave valve, 8. deltoidea, the 

 striae are not regularly alternate, but are irregular, and generally on the New York 

 examples there are three or four, sometimes only two, strong striae together, or with one 

 or two finer ones between. The first shell, 8. incrassata, Saff., is not described in the 

 New York Reports from the Trenton, but from the Chazy, and I think it distinct from the 

 Trenton form. It has the area on the convex valve. On the same page with the 



