THE CHONDROCRANIUM OF LEPIDOSTEUS. 249 



The vidge forms the bounding edge between the dorsal and 

 lateral surfaces of the chondrocra,nium, and it may be that it has- 

 developed for this particular purpose, but there is the evident 

 suggestion that it has l)een developed in some relation to the 

 main latero-sensory canal, for that canal always lies parallel to 

 and slightly mesial to it, and the lateral edges ojf the deimal bones 

 that later enclose that canal are always supported by it. The 

 relations of the ridge to the ridge of the lateral semicircular canal 

 are then simply fortuitous. The ventral edge of the so-called 

 facet for the hyomandibula of Veit's 2O5 mm. embryo corresponds- 

 to the opisthotic ridge of my descriptions of Aonia, Scomber, and 

 the Mail-clieeked Fishes, and has no relations whatever to the- 

 articular facet for the hyomandibula. 



In the prepared skull of a la.rge specimen of Lepiclosteus 

 2jlcUostGmus, I find the slightly depressed region which, in Veit'& 

 149 mm. specimen, lies immediately posterior to the hind end of 

 the spheno-pterotic ridge, as a.n irregular and relatively deep 

 fossa, which lies mostly on the lateral surface of the cranium but 

 partly also on its posterior surface, the ventral portion of the 

 fossa cutting across the dorso-anterior end of the lateral occipital 

 ridge and its dorsal portion, which has pit-like depressions in its 

 floor, extending upward mesial to the hind end of the spheno- 

 pterotic ridge. T!ie lateral occipital ridge lies wholly on the 

 exoccipital, ending at the dorsal end of that bone, and mesial to- 

 its dorso-anterior end an entirely independent ridge begins, and^ 

 lying wholly on the epiotic, runs dorso-anteriorly toward the 

 summit of that bone. This latter ridge is thus the homologue of 

 the epiotic ridge of my descriptions of Amia, Scomber, and the 

 Mail-cheeked Fishes, and the lateral occipital ridge is the homo- 

 logue of that part of the lateral occipital ridge of the latter fishes 

 that lies venti\al to that process of the opisthotic that gives 

 articulation to the pedicel of the suprascapula. 



In the prepai-ed skull of a much smaller, but still adult spe- 

 cimen of Lepklosteus osseus, somewhat similar conditions were 

 found ; but in a second specimen of this fish, not previously dis- 

 sected, the lateral occipital ridge was found continuing upward 

 on to the ventral surface of tlie overhanging epiotic process, but 

 there double, the postero-mesial portion of the ridge corresponding 

 to the independent epiotic ridge of the other specimen. In this 

 second specimen there is a short but well-defined groove on the 

 lateral surface of the chondrocranium, anterior to the dorsal end 

 of the lateral occipital ridge. This groove lies external to the hind 

 end of the lateral semicircular canal, in the hollow between that 

 ridge and the ridge of the posterior semicircular canal, is seen in 

 posterior as well as la.tera.l views of the cranium, and leads, 

 dorsally into the hind end of the snpraotic depression. It 

 evidently corresponds to the ventro-posterior continuation of the 

 temporal groove of Amia, Sconber, and the Mail-cheeked Fishes, 

 but in Lepidosteus it lodges a part of the thymus, and it is un- 

 questionably this gland that is the cause of the marked clifierences. 



