Dr. R. H. Traquair — On the Genus MegaUchthys. 117 



Saurodipterine cranial osteology ; a few points may however be con- 

 veniently alluded to. 



1. The polygonal plates covering the ethmoidal region between 

 the frontals and the prsemaxillae are often more or less distinct in 

 MegaUchthys, as in small specimens of Osteolepis ; often they are 

 fused with each other, and with the adjacent bones named above, as 

 seems always to be the case in Diplopfei'iis. 



2. Though Agassiz made a singular mistake with regard to the 

 nasal openings of MegaUchthys, he was perfectly correct in recognizing 

 the anterior position of the orbit, and in assigning to it a situation 

 exactly corresponding to that in Osteolepis. Prof. Young, of Glasgow, 

 has, however, in a brief notice of a head of MegaUchthys belonging 

 to Mr. John Smith, Kilwinning, Ayrshire,^ stated that the two outer 

 plates of the posterior half of the cranial shield, which he calls 

 " anterior frontal " and " squamosal " (posterior frontal and squamosal 

 of the nomenclature adopted by myself), bound the orbit above. 

 This would certainly put the orbits into a position very diiferent from 

 that which they occupy in Osteolepis. By the kindness of Mr. 

 Smith, I have had an opportunity of examining the specimen in 

 question, and though I find that on one side there is in the position 

 indicated a triangular space formed by a displacement of the adjacent 

 cheek-plates, I fail to see how it can be interpreted as an orbit, 

 while, on the other hand, the position of the real orbit can, I 

 think, be readily enough recognized *in the place where we would 

 expect to find it. 



3. I have not seen in any specimen of MegaUchthys the foramen 

 which occurs between the frontal bones in Osteolepis and Diplopjterus, 



4. Although omitted in Miller's and Pander's figures, lateral 

 jugular plates are undoubtedly present in Osteolepis and Biplopteriis, 

 as well as in MegaUchthys. 



The microscopic structure of the teeth of the Old Eed Sandstone 

 Saurodipterines is not yet fully elucidated ; so far however as ex- 

 ternal shape goes, there is nothing of sufficient importance to exclude 

 MegaUchthys from the group. 



Then as regards the fins. The Saurodipterini have obtusely 

 lobate pectoral and ventral fins, two narrow dorsals, one similarly 

 shaped anal, and a caudal, which may be heterocercal (Osteolepis) or 

 diphycercal (Diplopterus) . Triplopterus of McCoy, supposed by him 

 to have only one dorsal fin, is a genus which is really non-existent, 

 as it was founded on a specimen of Osteolepis compressed in such a 

 manner as to show both ventral fins, one of which was mistaken for 

 the single dorsal. The dorsal fins vary in position in Osteolepis and 

 Diplopterus, being in the latter opposite the ventrals and anal re- 

 spectively, while in Osteolepis the first dorsal is in advance of the 

 ventrals and the second opposite tbe space between the ventrals and 

 the anal. Now we have already seen that the lobate form of the 

 pectoral in MegaUchthys Hibberti was not unknown ; it is noticed by 

 Mr. Ward,^ and was indeed incidentally alluded to long before by 

 Agassiz^ himself, in describing what he supposed to be the ventral 



1 Pr. G. S. Glas. iii. 1868, 202-3. - I.e. ^ Poiss, Foss. du vieux Gres rouge, G3. 



