SKELETON OF EEGALECUS AEGENTEUS. 7 



1. The Cranium. (Plate IV. figs. 6-10, and Plate V. fig. 1.) 

 As usual in Teleosts, the cranium consists of a compact mass of mingled bone 

 and cartilage, formed from the parachoidal, trabecular, and otic elements of the 

 primitive skull, with the addition of certain closely fitting membrane-bones. A con- 

 siderable amount of the original cartilage (coloured purple) is retained, especially in 

 the auditory, ethmoidal, and prenasal regions. The bones are very thin and spongy, 

 consisting of delicate plates and needles which shoot, almost like crystals, through thin 

 cartilaginous or membranous ground-work. So imperfect, indeed, is the ossification of 

 the cranium, as well as of other parts of the skeleton, that there is rarely any difficulty 

 in distinguishing, in the adult, between cartilage and membrane-bones. The former 

 are in all the figures, except that of the entire skeleton (fig. 5, PL III.), coloured 

 yellow. 



In consequence of the very irregular edges of the bones, the determination of their 

 boundaries was a matter of considerable difficulty, and in many cases was impossible 

 without actual disarticulation. As there was only a single specimen at my disposal, I 

 had the cranium bisected, after making the necessary sketches of it (figs. 7-10), and then, 

 having drawn the longitudinal sections (figs. 11 and 12), disarticulated the left half 

 (fig. 13, PL V). 



Compared with the facial part of the skull, the cranium is remarkably small ; its 

 form is very peculiar, having a curious superficial resemblance to the skull of a Cha- 

 mseleon. Its strange appearance is largely due to the fact that the basis cranii is 

 produced downwards into a thin vertical plate — the subcranial crest — the height of 

 which is considerably greater than that of the brain-case proper. As a result of this, 

 the parasphenoid (va.s) is carried ventralwards far below the proper level of the basis 

 cranii (fig. 11, b.cr), and the ethmoidal and prenasal regions (p.n) are bent down to 

 meet the vomer (vo), which, as usual, forms a direct forward continuation of the para- 

 sphenoid. This remarkable distortion of the cranium seems to be correlated with the 

 great size of the eye and the consequent necessity for an increase in the dimensions of 

 the orbits. In fig. 5 (PL III.) the sclerotic is shown in situ, and it is seen that although 

 the eye is by no means unusually large, as compared with the whole skull {cf. fig. 1), 

 yet it is immense in comparison with the cranium. 



Another noteworthy peculiarity is the absence of the large processes which are 

 usually so marked a feature of the teleostean skull. There is no trace either of supra- 

 occipital crest or of epiotic or parotic process ; there is, indeed, a fairly prominent blunt 

 elevation (figs. 8 and 10, b.tm) near the ordinary position of the parotic process, but 

 this is really formed not by a cranial bone at all, but by the posttemporal or dorsal- 

 most bone of the shoulder-girdle {vide infra, p. 26), which has not the usual forked 

 form, and is so closely applied to the dorsal surface of the auditory capsule as to 

 require some force for its removal. 



