part 2] SYRIKGOTHYRIS AND SPIRIFEIUNA. 205 



Horizon and localities. — Upper Dibunophyllum Zone (D„) 

 Wetton (Staffordshire) ; Castleton, etc. This form is more par- 

 ticularly characteristic of the ' Brachiopod-Bed ' type of deposit, 

 and its relation to Tylothyris subconica subconica recalls that 

 which exists hetween the forms of Syringothyris occurring in 

 normal deposits, and the special types found in the knoll-phase 

 limestones. 



Holotype (of I. subconica castletonensis). — A specimen from 

 Settle on Tablet 501, in the Sedgwick Museum, Cambridge. 



Comparison with Syringothyris cuspidata. — T. sub- 

 conica subconica, T. subconica castletonensis, and S. cuspidata 

 sometimes occur in the same beds, and have frequently been con- 

 fused. The difference between the first two and the last is, of 

 course, at once evident if the beak of the pedicle-valve is broken, 

 or is rubbed down ; but there are also external differences. Com- 

 pared to either of the species of Tylothyris with which it can 

 occur, S. cuspidata is a larger shell, has a higher area, more 

 numerous and less angular costae, and a less sharply-defined sinus 

 in the pedicle-valve. Moreover, it never exhibits the lamellose 

 ornament seen in suitably-preserved specimens of Tylothyris. 

 The shell recorded by Phillips as a ' remarkable variety of Spirifera 

 cuspidata ' was in all probability T. subconica. 1 



Comparison with T. laminosa. — The flat area of T. sub- 

 conica subconica at once distinguishes it from T. laminosa. 

 T. subconica castletonensis differs from that subspecies in its higher 

 area, the beak of which is not much raised above a line perpen- 

 dicular to the plane of the brachial valve. The fold is more 

 elevated, and the line along the floor of the sinus more strongly 

 curved in T. laminosa than in the later species. 



Certain shells occurring in the Dibunophyllum Zone which are 

 more or less intermediate between T. subconica castletonensis and 

 T. laminosa, but more closely resemble the latter species, are 

 included under that name, as no useful purpose would be served 

 by attempting to differentiate them. They include forms like that 

 figured by Davidson, 2 and the specimen from Bundoran mentioned 

 by him on p. 15<3 (see fig. 4 c, p. 199). This latter form from the 

 ' Calp,' which includes representatives of S., and D, is in all prob- 

 ability intermediate in age between T laminosa and T subconica. 

 Here also belongs a specimen referred to by Martin in his descrip- 

 tion of Anomites cuspidatus. He says : 



' In the other anomite which I have not as yet named or described the hinge 

 is still less than in A. su.bconicus, and the beak of the large valve instead of 

 being straight is somewhat incurved.' 



J. Pai'kinson, in his ' Organic Remains,' figured what he believed 

 to be the form to which Martin referred. His figure shows clearly 

 the curved area and the laminose surface-ornament. 3 



1 Phillips [27] p. 216. 



2 Davidson [4] p. 36 & pi. vii, figs. 21-22. 



3 Parkinson [26] p. 235 & pi. xvi. fig. 16. 



