312 Cambridge Philosophical Society. 



illustrations of the microscopic structures ; and acknowledgements 

 are made of the kind co-operation of the Kev. Prof. Clark and Mr. 

 Anthony of Caius College, Cambridge, in this part of the investiga- 

 tion, by allowing the use of their large microscopes, and assisting to 

 prepare the transparent fragments for examination. 



Twelve new genera are proposed : — 1st. Isodus, for a fish of the 

 yellow sandstone, having very numerous teeth with a simple conical 

 pulp-cavity in their upper part, which divides into branches below as 

 in Rhizodus (Owen) ; but the section is circular, and the teeth are 

 all equal in size. 2nd. Centrodus, for curved conical teeth with a wide 

 simple pulp-cavity, reducing the base to a sharp edge, and having 

 not only the form but the microscopic structure of a reptile tooth, 

 that is, from the simple pulp-cavity minute calcigerous tubes radiate 

 to the circumference, terminating near the surface in a layer of small 

 calcigerous cells, covered by a layer of true glass-like enamel, pre- 

 senting no trace of structure with a power of 300 diameters, and 

 quite distinct from that dense modification of dentine, which, forming 

 the polished surface of most fish-teeth, has been confounded with 

 true enamel, but which it is here proposed to call ganoine in future 

 descriptions. 3rd. Colonodus, for very long simple teeth with simple 

 pulp-cavity, and their sides indented by transverse wrinkles. 4th. Os~ 

 teoplax, for large, flat, polygonal dermal plates, minutely wrinkled on 

 the surface, and allied to Psammosteus (Ag.) of the Old Red Sand- 

 stone ; but while the latter plates are composed of horizontal layers 

 of large cells, the present genus has a very singular microscopic 

 structure, being traversed by vertical branched (Haversian ?) canals 

 terminating in the pores of the surface ; and in the intervening blas- 

 tema are numerous oval Purkinian cells, the radiating tubuli of which 

 do not anastomose. 5th. Erismaeanthus , for a singular Ichthyodo- 

 rulite not uncommon in the Armagh limestone, which, arising from 

 a large compressed base, branches into two portions, one long ante- 

 rior closely tuberculated prop-like portion, and another extruding 

 backwards, short, and resembling a small Ctenacanthus , but with 

 smooth ridges. 6th. Platycanthus, for small spines, extremely wide 

 and compressed, resembling small Oracanths, but arched and with 

 posterior rows of teeth. 7th. Dipriacanthus, for small, curved dorsal 

 spines, which have two rows of denticles pointing downwards on the 

 posterior face, and two rows pointing upwards on the anterior face, 

 reminding us of the recent Pimelodus and Synodontus of the Nile. 

 ■8th. Polyrhizodus, an extraordinary genus of Psammodontoid teeth 

 not uncommon in the Armagh limestone, having the root divided 

 into numerous fang-like lobes, as in a mammalian tooth. 9th. Glos- 

 sodus, for certain tongue-shaped teeth allied to Helodus (Ag.). 10th. 

 Climaxodus, for some palates allied to Poecilodus (Ag.), but instead of 

 being transversely trigonal and obliquely ridged, they are equilateral, 

 and have the ridges transverse and parallel (like a flight of steps). 

 11th. Chirodus, for little hand-shaped teeth allied to the Ceratodi, 

 but distinguished by the thumb-like lobe projecting from the middle 

 of the long side, and which would prevent the union of the teeth in 

 pairs in the mouth, in the manner of Ceratodus. 12th. Petrodus, 



