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Recherches sur les Poissons Fossiles. 



of Tetragonolepis. They are bifurcated in many, from the middle of 

 their length, and articulated very close to their insertion. The fulcra 

 which border the superior lobe are thick at the point of insertion of 

 the rays of the fin, but seem to diminish rapidly towards the extre- 

 mity ; those whicli are extended the whole length of the inferior lobe, 

 are very small at the base of the fin. 



Tetragonolepis dorsalis, Ag. PI. IV. The original from which this 

 plate was taken, M. Agassiz found in the British Museum, and is from 

 the lias of Gloucester ; numerous specimens have been found ; they are 

 very easily distinguished by their oblong oval form, and the elongated 

 rays at the anterior border of the dorsal. The head is short but large : 

 on the surface of the bones we remark certain ridges of a fine granular 

 series, particularly on the operculum. The thoracic girdle is also 

 granulated, but the scales are perfectly smooth ; the enamel of which 

 they are covered is so thin that we can see the edges of the laminae 

 of growth. They are of medium size, particularly on the sides and 

 parieties of the abdomen, but much smaller on the tail, and along 

 the back, their edges are straight and entire ; they are not deticulated 

 on their posterior border. The lateral line is very visible ; it is ex- 

 tended almost straight with only a slight bend towards the back from 

 posterior superior angle of the operculum, to near the middle of the 

 base of the tail. The dorsal commences a little in front of the mid- 

 dle of the back, and terminates opposite to the posterior extremity of 

 the anal fin, which is scarcely more than half its length. The ante- 

 rior extremities of the fins are more elevated than the posterior. The 

 anterior rays of the dorsal in particular, are about double the length 

 of the posterior. The largest are, however, preceded by some other 

 little rays which succeed one another in front, gradually diminishing 

 in length. The rays of these two fins are slender, bifurcated in the 

 upper third of the length, and have their articulations transverse at 

 the distances. The caudal is large in proportion, its insertion is 

 slightly oblique, and the rays of the superior lobe sensibly shorter, 

 their base being covered with the scales of the pedicle of the tail, 

 which is extended more in the superior than inferior lobe. The rays 

 of this fin are slender, and branched below the middle, and divided 

 into articulations like those of the dorsal and anal fins ; along 

 the upper and lower borders, we observe a number of small fulcrums. 



