ii6 



CHIM^ROID FISHES AND THEIR DEVELOPMENT. 



however, does not refer to these structures in connection with Chimsera (MT. Zool. 

 Stat. Neapel, vi, p. 221 ct scq.). In further detail: in the dorsal scales of Callo- 

 rhynchus, as Dumeril and others have shown, the individual scales are furcate at 

 their base, and the free points of the base project forward and embrace the pre- 

 ceding member of the series, thus rendering the row of scales stronger and more 

 compact (r/. esp. Dumeril, Garman, and Schauinsland). (Figs. 93 c-e. ) It has 

 further been shown by Schauinsland that these scales present notable shark-like 

 features in their development; they first arise, like shagreen denticles, as an out- 

 growth of the derma; they then differentiate odontoblasts, by which in a centrifugal 

 direction dentine is laid down; and at the end of the process a pulp cavit}^ remains 

 and a basal plate perforated by small nutrient canals. In Schauinsland's words we 

 further note that "in the latest embr3'onal stages the denticles, and especially their 

 tips, acquire a greater and glassy transparency (vitrodentine), by which they 

 become more and more differentiated from the substance of the (basal) plate. In 

 short, developmentally speaking, the dermal denticles of Callorhynchus represent 

 the most primitive scales which occur among living selachians. Through the 

 presence of a basal plate perforated by dentine tubules, they suggest the scales of 

 the oldest palaeozoic selachians."* Schauinsland illustrates his foregoing remarks 

 with two excellent figures, one showing in section an early stage (oj^. cit., Taf. xix, 

 fig. 139) in the development of the dermal cusp, the other a late stage in which 

 the cusp presents a thick cortical layer of vasodentine {ibid., fig. 140), projecting 

 its tip beyond the epidermis. 



On the basis of the foregoing observations, therefore, we may conclude that, as 

 far as these body scales are concerned, Callorhynchus is distinctly shark-like; there 

 is not the slightest embryological evidence that this Chimseroid had ever ganoid- 

 like scales. We might even, I think, go farther than Schauinsland, and point out 

 resemblance with more typical selachian conditions; for this author, while main- 

 taining that "the epidermis takes no part in the formation of the denticle," and 

 admitting that he "was unable to demonstrate the presence of enamel," shows 

 nevertheless in his earlier figure that the cells of the epidermis are arranged over 

 the dermal papilla in a wayf that is more than suggestive of an enamel organ — 

 an emphatically shark-like character; and we may further conclude that the base 

 of the denticle perforated with tubules is not merely characteristic of denticles of 

 Silurian forms but of later sharks as well {cf. Rose, re trabeculo-dentine in Anat. 

 Anz. , 1897, p. 36). In connection with the presence of scales arranged near the 

 dorsal line, it has already been commented on (Schauinsland) that these structures 

 are relatively more prominent in the late embryo than in the adult, although 

 no explanation of this phenomenon has yet been advanced. I may accord- 

 ingly hazard the opinion that they have been retained in this position owing to 

 their importance as larval organs — possibly for the purpose of enabling the well- 



*Cf. Kohon, J. O., ijber fossile Fische vom oberen Jenissei, Mem. .4cad. St. Petersburg, i88g, and Die ober- 

 silurischen Fische von Oesel, Mem. Acad. St. Petersburg, 1893. He refers to denticles of Thelodus-like forms which 

 the recent researches of Traquair have associated with fishes which are in some regards shark-hke. 



\Cf. e.g., Jentsch, B., Beitr. z. Entwick. u. Struktur d. Selachierzahne. Leip. 1897, fig. 6. 



