32 FIELD COLUMBIAN MUSEUM ZOOLOGY, VOL. i. 



that at sometime in the history of the larva, or during its ancestral 

 history, the base of the lower half-arch has fused with the little inter- 

 mediate cartilage which lay in front of it ; but I have seen no signs 

 of its presence. It has likely been suppressed. Now, as will be 

 readily conjectured, and as will be further demonstrted, the ''interca- 

 lated vertebrae" of the adult, or as we have called tjiem, tJie pleurocentra, 

 have originated from the four little masses of cartilage lying in front of 

 the arches belonging to each myomere; while the hypocentra have had their 

 origin from the union of the upper and lower arches. Furthermore, the 

 simple vertebra of the region in front of this are developed from the bases 

 of the lower arches and the basal cartilages on which the upper arches 

 rest. 



Again, there are none of the extra cartilages placed between the 

 bases of the upper and lower arches in the upturned region of the 

 tail. It is possible that, on account of the crowding together of 

 structures in this region, these nodules never made their appearance 

 there. Or, being possibly younger productions, they were the first to 

 feel the effects of influences leading to degeneration, and at length 

 disappeared. 



Before we investigate the later history of the arches and the 

 accompanying cartilages, it will be well to inquire whether anything 

 corresponding to the latter occur elsewhere, especially in related 

 lower forms. These are found in a position entirety similar to what 

 are called intercalated cartilages in sharks. Whether or not they are 

 homologous structures in the two forms I do not here decide. But 

 similar intercalated cartilages are found also in the cartilaginous Gan- 

 oids. Fig. it has been drawn from the dorsal region of Acipen- 

 3er. Here we have upper and lower arches resting by their bases- 

 on the sheath of the large and partly exposed notochord. Between 

 the bases of the upper arches are seen intercalated masses of cartilage, 

 each mass consisting of from one to four pieces. Between the ex- 

 panded bases of each two of the lower arches is also a group of car- 

 tilages varying in number, size and form. These cartilages were 

 long ago observed by V. Baer and J. Muller. (V. Baer, 2; J. Muller, 

 49, p. 87.) While these intercalated cartilages are thus broken up in 

 Acipenser into a number of distinct pieces, we find that in Polyodon 

 (Spatularia) each, "both above and below, consists of a single piece. 

 See figure of a portion of the vertebral column of Spatularia in 

 Wiedersheim's " Vergleichende Anatomic der Wirbelthiere." 



If now, in either Acipenser or Polyodon, the upper intercalated 

 masses were to increase in size to any considerable extent in an ante- 

 rio-posterior direction, the bases of the upper cartilages would be 



