SKELETON OF EEGALEOUS ARGENTEUS. 11 



of cartilage very poorly ossified. Each has a nearly straight anterior and a curved 

 posterior border ; articulates in front with the sphenotic (sp.o) and frontal (/>'), below 

 with the alisphenoid (als) and opisthotic [op.o^], while behind it is separated from the 

 ascending process of the opisthotic [op.oi^) and from the epiotic by a mass of cartilage 

 produced on the cranial side into a large projecting base (figs. 11 and 12). The small 

 size of this bone is very unusual ; according to Stannius ' it always unites with its fellow 

 of the opposite side in the middle ventral line, so that the wide separation of the pro- 

 otics in Megalecus appears also to be highly exceptional ". In most Teleosts, also, the 

 bone under consideration is grooved or perforated in front for the exit of the fifth nerve ; 

 in the present case the nerve apparently passes out in front of the alisphenoid {vide 

 iyifm). It is further very usual for paired laminae of the prootic to form the sides of 

 the canal for the ocular muscles ; in Regalecus, as stated above, it is mainly the opis- 

 thotics which bound the functional representative of this canal. 



The pterotics (pt.o) are the largest of the otic bones, ossifying a considerable part of 

 the dorsal and external regions of the auditory capsules. Eacli pterotic consists of two 

 portions ; a flat plate, which articulates behind with the exoccipital, in front with the 

 prootic, and below with the opisthotic ; and an elongated, forwardly directed portion, 

 which forms the dorso-lateral boundary of the auditory region, and articulates on its 

 inner side with the epiotic and parietal, while in front it is wedged in between this 

 parietal, frontal, and sphenotic. 



The sphenofics {sp.o) form, as usual, the postorbital processes. Each is overlapped in 

 front by the frontal, articulates by its inner border above (fig. 8) with the pterotic, below 

 (fig. 9) with the alisphenoid ; by its posterior edge it joins the prootic. Below it sends 

 off a strong descending process, which, articulating with an ascending process of the 

 parasphenoid, forms the postorbital pillar {p.or.p). 



The alisplienoids {als). — Immediately in front of the prootic, and below the sphenotic, 

 is a flat bone with a straight ventral and a curved dorsal border, and having a very 

 evident groundwork of cartilage. It forms the lower moiety of the skull-wall imme- 

 diately in front of the auditory region, and, with its fellows, bounds the large foramen 

 marked A in the figures. It articulates by the whole of its dorsal edge with the 

 orbital plate of the frontal (fig. 12, fr^) on the inner face of the skull-wall, with the 

 sphenotic (fig. 9) on its outer face. The sphenotic overlapping the frontal in this 

 region, its anterior border articulates with the frontal and the orbitosphenoid {o.s), and 

 its posterior border with the prootic and anterior process of the opisthotic. 



It is evident that this bone has, on the whole, the relations of the alisphenoid, and, 

 , as far as I can see, the only difficulty in the way of the interpretation is the fact that 

 there is no foramen for the fifth nerve behind it, and that, as far as one can judge 



' Op. cii. p. 58. 



' In Parker and Bettany's ' ilorphology of the SkuU,' p. 66, the union of the prooties in the Salmon is spoken 

 of as a " most interesting fact." 



c2 



