196 DR. F. J. NORTH ON [vol. lxxvi, 



Deltliyrium nearly twice as high as wide, delthyrial angle about 

 35°. Delthyrial covering unknown, although its former existence 

 is indicated by grooves on each side of the delthyrium. 



Pedicle-valve sub-pyramidal, lateral slopes convex or nearly 

 flat ; mesial sinus wide, subangular, and sharply differentiated 

 by the subangular costas on each side of it. 



Brachial valve gently convex, mesial fold distinct, and raised 

 above the general level of the valve, especially towards the anterior 

 margin. 



The lateral slopes in both valves are each ornamented by sub- 

 angular cost*, decreasing gradually in size towards the cardinal 

 extremities. Surface of both valves crossed by well-defined, regu- 

 larly-disposed imbricating lines of growth, which are on the average 

 about half a millimetre apart. Shell-structure fibrous and im- 

 punctate. 



Internal structures: — Pedicle - valve. — Delthyrial sup- 

 porting-plates divergent towards the floor of the valve. Median 

 septum well developed, reaching nearly to the level of the area at 

 the apex, becoming gradually lower and disappearing about half- 

 way between the beak and the anterior margin of the valve. The 

 earlier-formed portion of the median septum is embedded in the 

 apical callosity, which near the beak completely fills the cavity 

 between the delthyrial supporting-plates, and extends forward along 

 the floor of the valve, connecting those plates with the median 

 septum, on each side of which, at the termination of the callosity 

 about half-way along the median septum, there is a pit-like de- 

 pression which o-ives a very characteristic appearance to the interna: 

 cast. (See text- figs. 1 m& 1 n, p. 166. and PI. XIII, fig. 12.) 



Brachial valve. — There is a low median crest or septum. 

 Brachial skeleton and jugum imperfectly known. 



Range. — Carboniferous Limestone, Cleistopora Zone to Upper 

 Dibunophyllum Zone. 



(lenotype. — Tylothyris laminosa (M'Coy). 



Systematic position. — The systematic position of Tylothyris 

 cannot be stated with absolute certainty. The median septum is 

 only a special development of a feature represented in most of 

 the Spirifers, and cannot be regarded as of more than generic 

 value. 



The absence of punctse in the shell-substance connects it with 

 Spirifer rather than with Spirifer ina. 



The spiral coils, as seen from the rubbing-down of the brachial 

 valves of several specimens, are arranged with their apices directed 

 laterally outwards, that is, in the way normal to Spirifer and allied 

 genera. There is no evidence to suggest that the coils Avere spinose. 

 as in the Jurassic species of Spiriferina. Owing to the mode of 

 preservation of the shells, indubitable evidence of the precise nature 

 of the jugum has not been obtained. Small fragments of shelly 

 rod were observed in some specimens in the position in which the 

 jugum would be expected to occur. They were, however, obviously 

 disturbed, and it was not possible to determine whether the}' 



