SHARK-LIKE DERMAL DENTICLES. II7 



developed young to maintain its position in tlie egg-capsule, possibly also for the 

 purpose of protecting the delicate dorsal fin, i. c. , by keeping it from rubbing against 

 the walls and the roof of the capsule, during the movements of the young fish. 

 According to this view the dorsal scales of the young Callorhynchus after the time 

 of hatching are to be looked upon merely as rudimentary organs.* And it may 

 be pointed out, in this connection, that when these enlarged dorsal scales are 

 developed in shark embryos they appear o)ily in those forms in ivhich development 

 takes place in egg-capsules.^ 



(b) Small dermal plates have long been known to occur in Chimseroids in 

 connection with the sensory-canal system. Pollard makes a special reference to 

 those situated in the suborbital canals, and Schauinsland gives the following notes 

 upon them (op. cit., p. 13): 



In the immediate neighborhood of the mucous canals — I have investigated those only situated 

 on the head — there also occur dermal calcifications. I find there (in transverse section) in the 

 floor of the canal (in the neighborhood of the skull) a large plate, and in addition at its sides and 

 bounding it four to six conical caps of dentine. The development of these is like that of the 

 denticles, save that the plate contains no pulp cavity, while the lateral small hard structures 

 present such a cavity, if indeed only in a narrow form, and filled with few cells, whereby they 

 come to resemble a small denticle. These calcifications are also probably only the rudiments of 

 former dermal denticles which came to sink down at the same time that the epidermis was invag- 

 inated to form the mucous canals ; in this process they lost their primitive form and underwent 

 degeneration. In adult, and especially in a number of fossil Holocephali the slime canals are 

 surrounded by a great number of closely compressed rings formed of calcified and bony material ; 

 these had their origin through a process of pressing together the single dentine-like bony caps 

 noted in the embryo. 



In the matter, then, of the character of these plates in living forms, we may again 

 conclude that they are equally derived from solitary dermal denticles, shark-like in 

 type. There is no evidence, on the side of embryology at least, that these plates 

 result from a breaking down of larger structures. It is only necessary to note 

 further that these structures in Callorhynchus are most marked in their likeness to 

 the selachian condition, and that they are least marked in the case of Chimsera. % 



(r) In all recent Chimasroids numerous denticles are present in the male, /. e. , 

 on the frontal clasping organ, on the mixipterygium, and on the anterior pelvic 

 clasping organ. These denticles have a transparent, almost glassy character. In 

 the frontal clasping organ of Callorhynchus, they occur not only at the tip of the 

 organ itself, but also proximalward and at the front and sides of the depression into 

 which this clasping organ fits; but in the other genera, the denticles are limited 

 only to the tip of this organ. It follows, accordingly, that in Callorhynchus appears 

 again a more shark-like character, /. e., a greater number of denticles spread over 

 a larger extent both of the clasping organ itself, and of the sheath into which the 



*In a specimen of Cal/or/tynciii/s " antarclicus" (Australia), measuring 92 cm. in length, the dorsal denticles 

 have disappeared. 



fThe tubercles in the encapsuled Scyllium (de Philippi, Paul Meyer) may well have a similar function. By 

 Paul Meyer they are described (o/. cit., p. 224) as rudimentary organs, viz., the remains of the ancestral annelidan 

 parapods! 



XKs to the condition of these dermal elements in fossil Chimaeroids, v. figs. 138 and 139 ; by evidence thus obtained 

 the conclusion becomes definite, i. <■., that the shagreen of recent forms has been greatly reduced from a condition 

 altogether shark-like. 



