284 Mr. A. S. Woodward — Pa'UeicJdliyoIogical Notes. 



posterior face appears slightly convex when viewed in trans- 

 verse section (figs. 5 h, c). Towards the distal extremity of 

 the spine the hinder face also exhibits a small, sharp, longi- 

 tudinal, median ridge (fig. 5 a) , and it is between this ridge 

 and the lateral borders that the diminutive pointed denticles 

 are placed, at wide and not very regular intervals. 



The Triassic fin-spine thus agrees with those of the Mesozoic 

 Hybodonts, and differs from all known Paleozoic spines''^ in 

 having the posterior denticles within the postero-external 

 margin. The sharp median ridge, however, is quite different 

 from the broad elevation of the posterior face in Hybodus^ 

 AcroduK, and Asteracanthus, and the denticles do not approach 

 the median line so nearly as is usual in the latter genera. 

 Leiacmithus may therefore eventually pi'ove to be a valid 

 genus, especially if the hollow ^c/Wws-shaped teeth commonly 

 ascribed to '■^Hyhodus heuperinus " are correctly placed as 

 part of the same fish. 



Form, and Loc. Upper Keuper, Shrewley, Warwickshire. 



2. On Nemacanthus raonilifery/*(??;i the Rhcetic Formation. 



The fish-remains from the Kh^etic bone-beds both of Britain 

 and the continent are usually so much broken and abraded 

 that their determination and description is not very satis- 

 factory. Occasionally, however, better preserved specimens 

 occur in the associated limestones and shales, and features 

 readily lost by abrasion are then observable. Among other 

 fossils, the Elasmobranch dorsal fin-spines named Nema- 

 cantlms monilifer are met with in especial abundance as 

 abraded fragments ; and since a diagrammatic section of one 

 fo these hfis been published to support an untenable theory f, 

 it seems advisable to offer a brief illustrated description of a 

 fine spine in Rhfetic limestone in the British Museum. 



The fossil in question is shown of the natural size in PI. X. 

 fig. 6, and is exposed from the right side. The large ganoid 

 anterior keel is preserved along the greater part of the 

 exserted portion, and there is no mark on the fibrous-looking 

 lateral face dividing the latter from the inserted basal portion. 



* Every Palfeozoic spine, with posterior denticles, hitlierto described 

 by American and European authors exhibits these denticles on the 

 postero-external mai-uin, as in the modern Chitncera. Every sufficiently 

 well-preserved specimen that the present writer has examined with 

 reference to this featui'e confirms the published descriptions. The unique 

 specimen on which Dr. O. Jaekel founds a diagrammatic section contra- 

 dicting all other authors (Neues Jahrb. 1892, vol. i. p. 140, fig. b) still 

 remains to be described . 



t O. Jaekel, Neues Jahrb. 1892, vol. i. p. 146, fig. a. 



