276 PROCEEDINGS OF THE GEOLOGICAL SOCIETY. [May 18, 



to consider liis specimen irreconcileable with the genus Dapedkis, 

 was the absence of the pecuhar processes by which the scales of most 

 of the Lepidoid fishes are locked to each other. Agassiz considers 

 him misled in this respect, since the scales are only seen outAvardl}'^, 

 in which case this characteristic mechanism would be concealed by 

 the adjoining scales ; but he does not deny the value of the character, 

 shoiild such be shown to exist. In fact, experience has proved this 

 to be one of the most valuable and trustworthy features with which 

 we are acquainted in the study of fossil ichthyology. On comparing 

 the scales of Mr. Brodie's species, which I have named Tetraffonolepis 

 discus, with those of Tetraffonolepis subserratus, with a view of de- 

 termining this point, it not only appeared that the ordinary lepidoid 

 mechanism was not present, but that the arrangement and articula- 

 tion of the scales coincided precisely with that found in the Pycno- 

 dontidcB, and in that family alone. 



This peculiar structure (Plate XI. fig. 3) is thus described in a 

 notice I communicated to the Society in 1849, on the afiinities of the 

 genus Plahjsomus : — " Each scale bears upon its inner anterior margin 

 a thick solid bony rib, extending upwards beyond the margin of the 

 scale, and sliced off obliquely above and below, on opposite sides, 

 for forming splices vdth the corresponding processes of the adjoining 

 scales. These splices are so closely adjusted, that without a mag- 

 nifying power, or an accidential dislocation, they are not perceptible. 

 When in situ and seen internally, these continuous lines decussate 

 with the true vertebral apophyses, and cause the regular lozenge- 

 shaped pattern so characteristic of the Pycnodont family." 



This description is precisely applicable to the specimens I have 

 alluded to ; in short, the dermal characters of Tetragonolepis are 

 closely allied to, if not identical with, those of the Pycnodontidce. In 

 some respects the lepidoid characters prevail : the form and position 

 of the dorsal and anal fins, the square tail, and the arrangement of the 

 opercular bones are very similar to these conditions in Dapedius. 

 The pectoral fin is situated opposite the junction of the operculum 

 with the suboperculum, a higher position than is found either in the 

 Dapedioids or the Pycnodonts ; the lateral line is straight, and pierces 

 the row of scales immediately above the spinal column ; the head is 

 exceedingly small ; the jaws are short and solid, and have not the 

 prognathic character so prevalent with the Pycnodonts ; the front 

 teeth are elongated cones with single points, one row alone is visible. 

 The true position of this genus will depend on the discovery of the 

 interior of the mouth, which is not seen in any of the specimens I 

 have examined. If it should prove that it is furnished with tritoral 

 teeth, there will remain no doubt of the propriety of assigning it to 

 the Pycnodont family ; in the mean time it must be considered as 

 having close affinities with it, and presenting a transitional form 

 between that family and the Lepidoidei. This genus then will com- 

 prehend the typical species Tetraffonolepis semicinctus, Bronn, Te- 

 traffonolepis subserratus, Miinster, Tetragonolepis discus, from the 

 lias of Gloucestershire, and a second new species from the lias of 

 Ohmden, which I propose to name Tetragonolepis cyclosoma. 



