284 Mr. A. 8. Woodward—Paualeichthyological Notes. 
posterior face appears slightly convex when viewed in trans- 
verse section (figs. 5, c). Towards the distal extremity of 
the spine the hinder face also exhibits a small, sharp, longi- 
tudinal, median ridge (fig. 5a), 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, 
Acrodus, and Asteracanthus, and the denticles do not approach 
the median line so nearly as is usual in the latter genera. 
Leiacanthus may therefore eventually prove to be a valid 
genus, especially if the hollow Acrodus-shaped teeth commonly 
ascribed to ‘“Hybodus keuperinus” are correctly placed as 
part of the same fish. 
Form. and Loc. Upper Keuper, Shrewley, Warwickshire. 
2. On Nemacanthus monilifer from the Rhetic Formation. 
The fish-remains from the Rheetic 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- 
canthus monilifer are met with in especial abundance as 
abraded fragments ; and since a diagrammatic section of one 
{o these has been published to support an untenable theory T, 
it seems advisable to offer a brief illustrated description of a 
fine spine in Rheetic limestone in the British Museum. 
The fossil in question is shown of the natural size in Pl. X. 
fie. 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 Paleozoic spine, with posterior denticles, hitherto described 
by American and European authors exhibits these denticles on the 
postero-external marvin, as in the modern Chimera. Every sufliciently 
well-preserved specimen that the present writer has examined with 
reference to this feature 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. 146, fig. 6) still 
remains to be described. 
+ O. Jaekel, Neues Jahrb. 1892, vol. i. p. 146, fig. a. 
