SKELETON. 127 



that the dental plates of the adult are attained only after a process of metamor- 

 phosis, during which the marginally high, delicate, glassy, and secant plates of the 

 young are worn down and give rise to the adult dental plates, broad and thick, studded 

 with tritors. Obviously, therefore, if we accept the view that a larval dentition is 

 present, it is clear that the substance of the dental plate can better be regarded 

 as a "precocious segregation" of the basal elements of teeth, i. e., along the outer 

 marginal rim of the plates, than as a new and independent accession to the 

 materials of development. All will admit, however, that the requisite proof of this 

 conclusion can be presented only by paleontology. On a later page the evidence 

 in this regard is summarized. 



SKELETON. 



The vertebrate column of Chimaeroids represents, according to Hasse (1879) a 

 polyspondyly, which he regards as typifying the ancestral condition in sharks. The 

 column of Callorhynchus was examined from the standpoint of embryology by 

 Schauinsland, whose conclusions I summarize as follows : 



That the early growth of the chordal sheath resembles that of many sharks, 

 inasmuch as its substance is invaded gradually, and only at few points, by mesen- 

 chyme cells. That cartilage appears quite late in development. That in each 

 segment (metamere) appear both neural and interneural plates, as well as corre- 

 sponding (i. e., double) haemal arches, especially throughout a greater portion of 

 the tail region. That these cartilaginous arches do not grow around with their 

 bases the secondary chordal sheath; this is only overgrown by a stout sheath of 

 connective tissue; the latter together with the arches on the one hand and the 

 secondary chordal sheath on the other forms the secondary vertebrae, but the 

 secondary chordal sheath is not divided into separate (primary) vertebras the 

 segmentation of the column being indicated only through these parate arches. 



Schauinsland, in brief, has been able to find no vertebral centra, in the sense 

 in which they occur in other fishes ; and my own studies upon Chimasra have been 

 no more successful in this important quest. No centra are found in either early or 

 late ' ' larval ' ' stages. Nor do they occur, as I suspected they might, after the 

 fashion of gerontic structures, in very large individuals. At the most, in the latter 

 case, there was a fusion of neural and haemal arches occurring in the region near 

 the occiput, but nothing which could be interpreted as definite centra. There is 

 still, none the less, the possibility that some form of centra were represented in the 

 ancestral Chimaeroid, and that they were gradually lost in ontogeny; indeed, as we 

 shall later note in the Jurassic Squaloraja and Myriacanthus, centra appear to have 

 been present in the anterior region of the column (figs. 138 and 140 c), where in all 

 recent Chimaeroids, indeed, the most perfect neural and haemal supports appear. 



The development of the skull has already been illustrated in several stages of 

 Callorhynchus by Schauinsland, and in a single late stage of Chimaera by the 

 present writer. The results of their observations are briefly these : The chimaeroid 

 cranium, instead of developing as a uniform trough-like brain-case (shark), appears, 

 even in early condition, in a wonderfully complete form ; it incloses the hindbrain, 



