278 PROCEEDINGS OF THE GEOLOGICAL SOCIETY, [May 18, 



in the British Museum, as also in the collection of the Earl of Ennis- 

 killen and in my own. 



3. T. CYCLOSOMA, mihi. This is a smaller species than the fore- 

 going. The body is about the size of a five-shilling-piece and almost 

 as round. The head is small, and the facial line coincides with the 

 general circular outline of the body. In T. snbsetratns the jaws 

 project beyond the confines of a circle described in like manner. The 

 depth of the body below the vertebral column is three times as great 

 as the dorsal portion above the column. The thoracic arch in this, 

 as in the other species, has a sigmoid flexure quite peculiar, the lower 

 half below the insertion of the pectoral fin sweeping downwards 

 and backwards with a considerable curve ; the fins and tail are all 

 deficient in the specimens I have seen ; the scales are seen internally, 

 the articulations characteristic of the family being very evident. The 

 single line of ventral scales is finely serrated, as in the preceding 

 species. 



Locality. Lias of Banz. 



The only specimens I have seen are in the collection of the Earl of 

 Enniskillen and in my own. 



4. T. DROSERUS, mihi. This is the specimen alluded to in the pre- 

 vious part of this memoir as being in the National Collection. It is 

 a fine species, nearly as large and much resembling in form the 

 Kupferschiefer Platysomi. The body is not nearly so deep in pro- 

 portion to the length, as in the other species. Most of the scales are 

 displayed internally, showing the articulating rib to be very strong. 

 Some are removed, leaving their impressions on the shale ; and some 

 few show the peculiar character of the surface, from which I have 

 taken the specific name. The appearance is granulated, but the 

 granules of ganoine scattered on the surface have so much more 

 lustre than the general superficies of the scale, that they resemble a 

 fine sprinkling of liquid. The teeth are very small compared with 

 the size of the fish (Plate XL fig. 4) ; the anterior ones are conical, 

 as in the genera Gyrodus and Microdon, and the succeeding ones are 

 short and thick, with a corrugated crown resembling the tritoral teeth 

 in the latter genus. The single row of abdominal scales is present 

 here, as in all the species of all the genera of the Pycnodont family, 

 and are coarsely serrated. This row of scales constitutes an im- 

 portant part in the mechanism of the dermal skeleton of these fishes, 

 for from each of them originate the two strong ribs which traverse 

 each row of scales and link them together ; so that in fact each of 

 them is the keystone of an inverted arch similar to the keel of a ship. 

 This mechanism is completed by a corresponding row of scales along 

 the back in most of the Pycnodonts, but it is not well seen in any 

 of the Tetrayonolejjides I have as yet examined. 



Locality. Lias of Boll. 



5. T, DISCUS, mihi. This beautiful little fish (Plate XL fig. 5) 

 was discovered by the Rev. P. B. Brodie in the Upper Lias of 

 Gloucestershire. The best specimen is perfect as far as the anal 

 region with the exception of the fins, which are deficient. It shows 



