1886. J PROCEEDINGS OF UNITED STATES NATIONAL MUSEUM. 75 



The uew features which appear in the ditfereutiatioii of the rays of 

 these forms are four iu number, viz : 



(fl) The atrophy of a unmber of actinotrichia at the bases or ahiiig 

 the whole width of the fin-folds, as a result of which absolutely eradi- 

 ate iuterradial spaces are formed. 



(b) The coalescence of a number of actinotrichia and their fusion within 

 a membranous matrix iu order to form a basis for the ossification of 

 the "soft rays" or malacopterygian type of tin ray. 



(c) The dichotomy of the soft rays due to the manner iu which the 

 actinotrichia are fused and drawn together anteroposteriorly at their 

 proximal ends. 



{d) The segmentation of the soft rays, the segments increasing iu 

 number with age, so that it seems that the segmentation is due to a 

 kind of transverse fracture during their development due to use in 

 swimming, as would seem to be indicated by a microscopic examina- 

 tion of the articulating ends of the segments, which seem to be widened 

 terminally by the pressure on alternate sides brought to bear upou 



them. 



Another advance is made in the development of the acanthoptery- 

 gian type of fin-rays or "spiues," which are mainly confined to the Phy- 

 soclistous types of Teleosts, where they develop iu more or less clearly 

 distinct epiblastic pockets in advance of the continuous fold which 

 gives rise to the malacopterygiau, dichotomous type of rays. Good ex- 

 amples of this style of development of spinous rays are seen in Lopliim 

 and Gasierosteus. It is doubtful if spinous, simple rays are developed 

 from actinotrichia at all, but from membrane formed by the mesoblast 

 and molded upon the inner walls of epiblastic pockets. 



While the Teleostean series, as a rule, develops a lophocercal stage, 

 there are notable execeptions, and it frequently happens that when the 

 median continuous fold is formed the actinotrichia are not developed 

 simultaneously throughout its whole extent. Only in types which are 

 apparently primitive, such as the Salmon, do the actinotrichia of the 

 entire fold develop so as to be visible along its whole extent. But the 

 continuity of the fold is soon interrupted by the atrophy of the inter- 

 vals between the median fins. 



It is thus made obvious to the discrimiuating student that embrj'- 

 ology is just as capable of supplying data of taxonomic value as a study 

 of outwardly palpable features or as a study of the anatomy of a series of 

 forms. But the obvious disadvantage under wdiich anatomy labors is 

 that it cannot safely surmise what genetic relations are indicated by 

 the morphology of the completed or adult organism, for the reason that 

 it cannot indicate the order and method according to which the various 

 parts made their appearance. The bluiulering on the part of anato- 

 mists in this respect is notorious, and is only excelled by the careless 

 taxonomist who is in search of difterences for the sake of discriminat- 

 ing new species, while he is in absolute ignorance of how such difter- 



