246 MR. E. p. ALLIS ON THE OTIC REGION OF 



splieno-ptei'otic ridge and in the line prolonged of the lateral 

 edge of the epiotic bone. The lateral portion of this sulcus, 

 lodges that thickened lateral portion of the dermopterotic that 

 is traversed by the main latero-sensory canal, and its deepest 

 portion lies between the ridges of the anterior and posterior 

 semicircular canals. The sulcus runs posteriorly over the hind 

 ed^e of the ridge of the lateral semicircular canal, and is there- 

 in communication, across the ridge of the posterior semicircular- 

 canal, with a slightly depressed region which lies directly beneath 

 the overhanging lateral edge of the epiotic and extends ventrally 

 to the dorsal edge of the vagus foramen. This depressed region- 

 doubtless lodges a part of the thymus, for, in an 80 mm. specimen 

 of this fish, I find a large anterior portion of that gland lying- 

 close against the cranial wall, immediately postero-lateral to the 

 descending limb of the posterior semicircular canal, and having a 

 small dorso-anterior prolongation which passes upward postero- 

 mesial to the levatores arcuum branchialium and the adductor 

 and levator operculi, at their insertions, and then over the -hind 

 edge of the spheno-pterotic ridge on to the dorsal siirface of its- 

 hind end. 



The depression for the thymus, above referred to, is separated 

 from the posterior surface of the chondrocranium by a sharp- 

 ridge, which Veit calls the crista occipitalis lateralis, this name 

 evidently having been adopted from Gaupp's ('93) descriptions 

 of Rana, where the so-designated ridge is said to be a band of 

 cartilage which forms the dorsal boundary of the vagus foramen, 

 and connects the otic capsule at the middle of its height with the 

 dorsal end of the occipital arch. The crista of Lepidosteits is thus 

 not the exact homologue of the crista of Eana, and would seem 

 to be a ridge secondarily developed upon that crista. It forms 

 the boundary between the dorsal and lateral surfaces of the occi- 

 pital portion of the chondocranium, and lies in Lepidosteus as in. 

 Rana wholly upon the os occipitale laterale (exoccipital), but in 

 many of the Teleostei the corresponding ridge lies as much upon 

 the basioccipital, opisthotic, and pterotic as upon the exoccipital. 

 In LepidosteAts it is said by Veit to vanish, anteriorly, on the roof 

 of the labyrinth region of the cranium, but it is not so shown in 

 his figures, there running directly into the ventro-posterior edge 

 of the epiotic bone, but separated from that bone by a narrow 

 space which has the appearance of being a notch in the edge of 

 the rido-e. The epiotic forms the dorso-postero-lateral corner 

 of the chondrocranium, and apparently has no relations whatever- 

 to the ridge of the posterior semicircular canal, for it is shown 

 lying definitely postero-mesial to the prominentia canalis semi- 

 circularis posterioris, Yeit says that he could find no indication 

 whatever of the epiotic being formed of two components, an 

 epiotic and opisthotic, as described by Parker ('82) in embi-yos 

 of this same fish. An opisthotic bone is, in any event, wholly 

 wanting in this fish, as is also, even in the adult, a.n autopterotic. 

 In a 20^ mm. embiyo of this fish, Yeit ('11, fig. 18, pi. c.) 



