1885.] PROCEEDINGS OF UNITED STATES NATIONAL MUSEUM. l59 



• 



authority states, of all other characters, except the scales, adequate to 

 separate the two. 



II. Faired Jins. — The paired fins of the salinou develop at wide inter- 

 vals from each other, that is, the ventral pair is separated from the pec- 

 toral by an interval of not less than sixteen muscular segments. In 

 Lophius the number of muscular segments opposite the interval be- 

 tween the pectorals and ventrals are reduced to four, according to the 

 figures given by A. Agassiz. In other types this interval between the 

 rudiments of the paired fins of the embryo is still further reduced, 

 as for example in Tracliimis vipera* and in Motella mustelai in which, 

 according to Brook, but two muscular segments intervene between 

 the earliest rudiments of the pectoral and jielvic fins, so that in 

 these types it may be said that the paired fins develope from almost 

 continuous rudiments. These and Lophius seem to develop the rudi- 

 ments of the paired tins almost synchronously, which is far from being 

 the case with tbe larvue of the Pliysostomous orders, Ginglymodi, Glan- 

 iostomi, Nematognathiy Plectospondyli, Isospondyli, Haplomi, and Unchel- 

 ycephali,^ in which the ventral pair of fins appears late, often after the 

 pectoral is well developed and in active functional use. In most, if not 

 in all, the larvai of Physostomous species of Actinopteri, as limited by 

 Cope, the ventral pair of fins is later in appearing than the pectoral ; 

 on the other hand in the Percomorph and Pediculate divisions of the 

 Physoclistous Actinopteri it seems that in many species the pectoral 

 and ventral limbs appear almost synchronously, the pectoral usually a 

 little more developed than the ventral pair, and separated serially by 

 only two to four myotomes, while this interval in the Physostomous 

 forms may embrace over sixteen muscular segments. 



Finally it may be said that the ventral pair of limbs is almost always 

 undergo shifting or translocation forwards in the Physoclistous groups 

 mentioned. The researches of Mr. Brook, cited above, afford additional 

 evidence of the truth of the principle laid down by me in a prelimi- 

 nary notice § recently published. 



The Physoclistous genera, Gadus, Cyhium [Scomheromoriis) and Pare- 

 phippus seem to be exceptions to the rule spoken of above, as holding 

 in the development of some forms. How far the rule held in the devel- 

 opment of the Palaeozoic fishes with ganoid scales and ventral or ab- 

 dominal pelvic fins, we, of course, now have no means of knowing. 

 These, with the exception of Dorypterus i\nd Blochius, seem to have had 



* Preliminary account of the development of the Lesser Weever-fish Trachinus 

 vipera, by George Brook, F. L. S., Jouru. Linn. Soc, xviii, pp. 274-291, pis. 4, 1884. 



t On some points in the development of Motella mustela, Linn., by Geori^e Brook, 

 F. L. S., Journ. Linn. Soc, xviii, pp. 299-307, pis. 3, 188.5. 



tThe eels are of course permanently without ventrals; the youngest obtainable 

 larvai of Anguillid;e and the Leptocephalid stages of marine eels show no traces of 

 ventral limb-folds: these forms are in fact permanently apodal. 



i On the Translocation forwards of the Rudiments of the Pelvic Fins in the Em- 

 bryos of Physoclist Fishes, Am. Najburalist, 1885, pp. 315-317. 



