THE CHONDROCRANIUM OF LEPIDOSTEUS. 253 



a conical depression on the posteiior surface of the cranium,, 

 which gives insertion to a portion of the trunk-muscles. 



The opisthotic is a relatively lar£>'e, concave-convex bone, the 

 concave surface presented posteriorly and the convex su}-face 

 antero-laterally, and it is difficult to comprehend how it could 

 have been wanting in Sagemehl's specimens of Moxosioma succetta. 

 The posterior portion of the postei'ior process of the pterotic rests 

 against the antero lateral surface of the opisthotic, near its dorsal 

 edge, and has there anchylosed with it, but the base of the 

 process does not touch the opisthotic, an opening there being- 

 left between the two bones and giving passage to the supra- 

 temporal branch of the nervus vagus. The opisthotic of this fish 

 thus corresponds to that part of the opisthotic of Scomber and 

 Scorpceiia that lies mesial to the lateral occipital ridge, this- 

 accounting for the fact that it lies against the mesial, instead of 

 the lateral, surface of the posteiioi- process of the pterotic. The 

 posterior process of the pterotic forms the floor of the lateral 

 opening of the temporal groove, this floor of the opening lying 

 at a considerably higher level than tlie floor of the remainder of 

 the gToove. The dorsal portion of the supraclavicula lies in a 

 somewhat vertical position, and its internal edge has a slight 

 antero- posterior sliding motion on the posterior portion of the 

 floor of the opening of tlie temporal groove, a curved process on 

 the dorsal edge of the supradavicula hooking over the edge of 

 the epiotic, from its anterior to its posterior surface, between the- 

 summit of the epiotic and a slight process mesial to it ; the- 

 supraclavicula thus completely closing the angular incisure in 

 the posterior edge of the temporal groove. 



The suprascapula of Sagemehl's descrij^tions lies, as he has 

 stated, along the posterior sui-fa,ce of that part of the epiotic that 

 forms the posterior edge of the temporal groove, and is without 

 pedicel, and neither it nor tlie supraclavicula come into contact 

 with th.e opisthotic. The suprascapula has no pedicel, and, 

 furthermore, is not traversed b}^ the main latero-sensory canal. 

 This bone of this fish thus cannot, as will be later further 

 explained, represent the whole of the similarly named bone of 

 Amici and most of the Teleostei, The absence of a pedicel to this 

 bone, together with the presence of a large opisthotic, is unusual, 

 for the two are usually associated with each other, a ligament 

 first developing between the suprascapula and the cranium, as 

 above described in Lepidosteus, this ligament then undergoing 

 ossification at its suprascapular end, and this ossification later 

 inducing ossification at its point of articulation with the cranium. 

 In Esox, where the suprascapula has a. well-developed pedicel, 

 the opisthotic is said to be wanting (Starks, '04), but in a single 

 specimen of this fish that I have examined I find it well 

 developed as a large membrane-bone loosely attached to the 

 cranium. 



The temporal groove of Moxostoma lodges a muscle which, 

 after it issues from the groove, lies posterior to the levator 



