BRITISH SILURIAN BRACHIOPODA. 145 



27. Triplesia (?) APicuLATA, Salter and Forbes, sp. Dav., Sil. Mon., H. XXV, fig. 6. 



Atkypa? apicdlata, Salter. Dav. Sil. Mon., p. 202, 1869. 



In my ' Silurian Monograph ' I placed this small shell with much uncertainty in the 

 genus Atrypa. It is evidently not a spiral-bearing Brachiopod, and may possibly be a 

 form of Triplesia. I regret not to have been able to examine more specimens, and am 

 unacquainted with its internal arrangements. 



28. Triplesia (?) incerta, Dav. Sil. Mon., PI. XXIV, fig. 30 ; PI. XXV, figs. 7 and 



8 ; and Sil. Sup., PI. VIII, figs. 24 to 29. 



Atrypa? incerta, Dav. Sil. Mon., p. 203, 1869. 



When describing this species at p. 203 of my ' Silurian Monograph ' I was acquainted 

 with the exterior only of the ventral valve. Since then Mrs. R. Gray and Dr. Callaway 

 have collected numerous specimens of the shell, some with both valves, which will 

 enable me now to complete its description, as well as to give figures of the interior of the 

 ventral valve. 



In external shape the shell is marginally obscurely subpentagonal, and about as wide 

 as long ; ventral valve deeper than the dorsal one, very convex, with or without a more or 

 less developed mesial fold ; beak either nearly straight or moderately incurved ; area 

 large, triangular, with a wide fissure arched over by a roof-shaped deltidium, divided 

 along the middle by a narrow convex ridge ; the area measures two or three lines in 

 width, the extremity of the beak sometimes tapering to an acute, narrow, and slightly 

 twisted termination ; dorsal valve convex at the umbo and lateral portions of the valve, 

 becoming gradually concave at a short distance from the umbo, and widening and 

 deepening as it nears the front. In the interior of the ventral valve the beak is, in some 

 specimens, much thickened, and more or less hollowed out in others. Two small con- 

 verging dental plates, in a specimen excavated by Mr. John Young, extend to the end of 

 the beak, when they diverge and continue to a short distance along the bottom of the 

 valve ; two pairs of muscular impressions, longitudinally parallel to each other and sepa- 

 rated by a short median groove, occupy a portion of the rostral portion of the interior of 

 the valve, and do not measure more than about two lines in length. We unfortunately 

 know nothing of the interior of the dorsal valve. The shell was in all probability not 

 provided with calcified lamellae for the support of the labial appendages. It is not a 



