BRITISH SILURIAN BRACHIOPODA. 143 



T. {?) Graya, Dav., and T. {? OrtJiis) spiriferoides, M'Coy, are provisionally referred to 

 Triplesia, as some uncertainty still prevails with respect to their generic characters. 



In true Trijjlesia the fold occurs on the dorsal valve, the sinus in the ventral one, 

 while it is the reverse in T. (?) incerta and T. (?) apiculata ; and other differences are also 

 observable, and which may perhaps necessitate their removal from Hall's genus. The 

 interior of these dorsal valves should be sought for. 



24. Triplesia m^v'Lkm^,d'Eichwald. Dav., Sil. Mon., PI. XXXVII, figs. 8—15 ; Sil. 



Sup., PL VIII, figs. 17 to 22. 



At p. 273 of my ' Silurian Monograph ' I described the external characters of this species 

 as an OrtJds, being at that period unacquainted with its interior arrangements. Since 

 then Dr. C. Callaway has been so fortunate as to discover at Ty-Isaf, in Wales, an 

 almost perfect interior of the dorsal valve, and has suggested to me that its generic 

 position would be with Triplesia. Some good internal casts were also procured by Mr. 

 Parrott from the Caradoc or Bala Shales of Cerrig Coedog, south-east of Moel Perna, 

 Corwen, Merionethshire, as well as by Professor T. McKenny Hughes from the Lower 

 Llandovery at Llettychyddod, Llandovery, and these will enable me to desciibe and 

 figure the interior of the dorsal valve. Triplesia insularis was also found by Mr. Robert 

 Philip in the Upper Llandovery at Minsterley, in Shropshire, in company with Pentaiaerus 

 oblonr/iis, P. rohmdatus, P. nndatus, and a number of other species that are common to 

 the Llandovery Rocks. No Wenlock Shales occur in the locality. 



In the interior of the dorsal valve there exists, close under the incurved umbonal 

 beak, a small projecting cylindrical cardinal process, broken at its extremity, so that it is 

 impossible to say whether or not when perfect it presented two deviating branches, as 

 in the American Triplesia Ortoni and T. extans. 



At a little distance from the anterior portion of the cardinal process, and at a slightly 

 lower level, are two small projecting tooth-hke projections, these and the cardinal process 

 leaving their corresponding holes or cavities in the internal cast. Under these three pro- 

 jecting processes there exists a wide W-shaped convex or swollen space, which at half 

 its length is divided by a deep depression, furrowed by a narrow median indented line, 

 bordered by wide protecting ridges. The quadruple adductor muscular impressions 

 are arranged in pairs, and occupy the central part of the bottom of the valve. In the 

 internal cast the convex space above described forms circular depressions, laterally 

 bordered by projecting margins. 



The ventral valve has a triangular area with a median fissure arched over by a pseudo- 

 deltidium, leaving a small open space close to the hinge-line for the passage of a 

 peduncle. 



20 



