part 2] SYBINGOTHYBIS AND SEIRIFERWA. 195 



Collection, is no longer in existence, or at least cannot be recog- 

 nized l ; (2) The various ways in which the fossils have been 

 preserved make their identification a matter of considerable 

 difficulty. Many specimens are silicified, and show only the 

 external appearance of .the shell ; others are more or less exfoliated, 

 and bear no trace of the imbricate structure which is an essential 

 feature of the species, while many are represented onty by internal 

 casts or external moulds. 



In the course of this investigation I have examined material 

 from various localities in the South- Western Province, and from 

 the Irish localities cited by M'Coy. Two Irish specimens pre- 

 served in the Sedgwick Museum, Cambridge, which were presented 

 to that Institution by Sir Richard Griffith and were named by 

 M'Coy himself, afforded most useful information : for one is a 

 testaceous specimen of a brachial valve of ' Spiriferina ' laminosa, 

 from Hook Head, and the other an exfoliated specimen of a 

 pedicle-valve from the ' Calp ' of Bundoran, 2 resembling those so 

 frequently found in the South- Western Province. 



The discovery of specimens showing a cast of the interior and a 

 mould of the exterior of the same shell, and a comparison between 

 wax impressions of such casts and the internal structures as 

 revealed hy sections across the apex of the pedicle-valve, have 

 ■established beyond doubt that the internal casts found in certain 

 bands of dolomite in the middle part of the Avonian also belong- 

 to the same species. 



The species under consideration differs from Spirifer in the 

 presence of a well-developed median septum in the pedicle-valve ; 

 from Spiriferina in the absence of the coarsely-punctate shell- 

 structure which characterizes that genus; and from Syringothyris 

 in the absence of a transverse plate and syrinx, in the possession of 

 a median septum, and. in the imbricate surface-ornamentation. 

 In the presence of a median septum accompanied by an impunctate 

 shell it resembles the Silurian genus Deltliyris (Dalman), but 

 differs from any other species referred to that genus in its much 

 greater size and its smaller and more numerous costse. and also in 

 the marked development of an apical callosity. There is, in the 

 circumstances, no alternative but to regard it as belonging to a 

 hitherto undescribed genus. It agrees in all its essentials with 

 'Spirifer' suhconicus (Martin), and I propose to assign those 

 two species to a new genus, Tylotliy ris. 



Tylothyius, gen. nov. 



Description. — Shell spiriferoid, about twice as wide as long, 

 greatest width along the hinge-line. , Cardinal area moderately 

 high, slightly concave or flat. Apical angle about 11.5°-120°. 



1 Dr. R. F. Seharff was good enough to institute a special search for this 

 specimen in the collection in the National Museum, Dublin, and it may be 

 definitely stated that it is no longer in that Institution. 



2 The register-numbers of these specimens are 635 & 636 respectively. 



