1 020 REPORT OF COMMISSIONER OF FISH AND FISHERIES. [40] 



forms, such as the Chimaeroids and Dipnoi^ in which the muscular aud 

 osseous segments are far less numerous than the homonomous rays, the 

 inclusion of the distal nodules spoken of cannot be so complete. Such 

 a condition also renders the muscular specialization much less marked 

 and more like that observed in embryo tishes at the time the embryonic 

 rays are formed. The erectores and depressores spinas muscles are 

 therefore not so distinctly differentiated, as at a later j^eriod, as must 

 obviously occur to the reader. 



True fin-rays, as found in the Lyrifera, are never preformed in carti- 

 lage, and, widely as they differ in extent of development, the actinosts, 

 as they have been called by Gill, as found in the pectorals of Teleosts, 

 are homologous with the greatly developed pro- meso- and metaptery- 

 gium of the Bays, together with the very elongate, numerous, often 

 dichotomous, multisegmented cartilaginous radii which they support, 

 provided they have in both cases been developed from the same num- 

 ber of serially homologous segments or somites, which are removed to 

 the extent of the same number of segments from the occipital somite. 

 This may be gravely doubted in some instances, as, for example, in the 

 case of Gastrostonms, where the pectoral in the adult is homonomous 

 with a segment or segments separated by at least fourteen from behind 

 and beyond the occiput. 



In the unpaired fins of Teleosts there is the clearest evidence that the 

 rays are homonomous with the somites, and that from one to five or six 

 rays are developed to a single somite, each ray involving the coales- 

 cence of a number of embryonic rays or fibrils which were dev^eloped 

 in the embryonic fin-folds. This concrescence of fibrils, the evidence 

 of which is given»in Plate IX, is found to occur in all of the fins of all 

 of the Lyrifera. 



A fin-ray of the lowest of the Lyrifera may be formed of only two 

 primitive fibrils to as many as twelve or even more, so that in the high 

 est types the greatest number of primitive fibrils or embryonic rays 

 enter into the formation of a permanent ray, so that the rays of the 

 highest aud lowest forms only differ in being respectively more or less 

 complex in this regard. 



The definition of a fin ray, which will hold for all of the Lyrifera, will 

 be as follows : A horn-like or osseous rod formed beneath the embry- 

 onic integument (epiblast), and at first composed of fibers, a greater or 

 Jess number of which become directly or indirectly blended to form a 

 permanent ray, lying distal of the axial cartilaginous or osseous sup- 

 port of the fin in the superficial mesoblast, and invariably consisting, 

 when ossified, of membrane bone, and always of more or less clearly 

 defined right and left halves, formed in the right and left sides of tbe 

 fin-fold, except when fused together into a hollow rod in the middle line 

 of an early stage. More superficial ossifications may become blended 

 with some anteriol" rays, as in Mematognathi, and lead to the develop- 



