398 MESSES. HANCOCK AND ATTHEY 



two genera ; and allusion was made to the fact that the Dipteri 

 are all small fishes, and that the Gtenodi, on the contrary are, 

 with one exception, all of considerable size. 



In the paper alluded to, seven species were described, six of 

 which were new. Three of the seven (namely, C. cristatus, C. 

 tuberculatum, and C. corrugatus) cannot have been less than five 

 or six feet in length. We originally estimated the length at four 

 or five feet ; but as larger specimens have since come to hand, 

 we now think that that estimate was too low. Three others 

 (namely, G. obliquus, C. imbricatus, and C. ellipticus) were pro- 

 bably upwards of three feet long. G. elegans is quite small. 

 The latter is the only species of which an entire specimen has 

 occurred ; and though much crushed and disturbed, its dimen- 

 sions can be determined with sufficient accuracy; it measures 

 only three inches in length, but, judging from the size of de- 

 tached dental plates, it probably reaches sometimes nearly 

 twice that length. The Dipteri are usually about five or six 

 inches long, and apparently never much exceed that length. 



The scales are very different in the two forms. "While in 

 Dipterus they are circular and truly cycloidal, in Gtenodus they 

 (PI. XIII., fig. 3) are elongated and parallelogrammatic, with 

 the posterior end well rounded, and the sides nearly parallel 

 or a little hollowed or concave ; they are in length nearly 

 twice their width, and, though imbricated, can scarcely be 

 called truly cycloidal : they are delicate and large for the size 

 of the fish, and are longitudinally ridged or grooved ; the 

 ridges, becoming curved and nodose, form a sort of rosette in 

 the centre of the exposed imbricated portion. This is very 

 different from the ornamentation of the scales of Dipterus, 

 which are either irregularly tuberculated, the tubercles being 

 elongated and scattered, or striated and punctate. The scales 

 alone would therefore seem sufficient for generic distinction. 



There are other characters, however, which distinguish the 

 two forms. When our paper on the subject was written, we 

 had not access to Pander's valuable "Monograph on the Cte- 

 nodipterini. ,,t ~ Since then we have enjoyed this advantage, and 



* " Ueber die Ctenodipterinen des Dcvonischen Systems," Dr. C II. Pander. 1858. 



