116 MR. E. PHELPS ALLIS, JUN., ON 



Between the mandibular articulai* facets of opposite sides, tlie 

 ventral surface of the chondrocraniuni is deeply concave, this 

 concavity lying in the ethmoidal region and being traversed by 

 the raised median longitudinal ridge of the fused trabeculte. 

 Anterior to tliis large subethmoidal depression there is a smaller, 

 subnasal one which lies in the hollow of the beak-like prenasal 

 process, the two depressions being separated by a V-shaped ridge, 

 each arm of the Y running antero-laterally from the projecting 

 ventro-posterior corner of the subethmoidal keel. The anterior 

 dental plates lie against the dorsal wall of the anterior half of 

 the subnasal depression, the posterior dental plates lying against 

 the anterior border of the subethmoidal depressioii. From the 

 bottom of the posterior half of the subnasal depression a tough 

 pad of tissue arises and projecting ventrally fills the space between 

 the dental plates. 



The bottom of the large subethmoidal depression is separated 

 by the raised ridge of the fused trabeculae into two parts, each 

 triangular in shape, and as each depression lies beneath the 

 lateral portion of the ethmoidal cartilage it may be called the 

 ectethmoidal depression, the two ectethmoidal depi-essions forming 

 the large subethmoidal one. Each ectethmoidal depression is 

 traversed by the nasal vein of its side, this vein, in some spe- 

 cimens, lying in a pronounced groove at the bottom of the 

 depression. This vein comes from the nasal capsule and the 

 regions anterior to it, and is directly continuous, in the nasal 

 capsule, with a vein that enters the cavum cranii, accompanying 

 the nervus olfactoriiis, and joins a vein that issues from the 

 cranial cavity through the foramen for the nervus vagus. The 

 nasal vein, running posteriorly from the nasal capsule, first 

 traverses a canal that leads from that capsule through the 

 anterior wall of the ectethmoidal depression, then receives a 

 branch that has traversed the overlying cartilage through a small 

 foramen, and then itself traverses a foramen in the postei'ior wall 

 of the ectethmoidal depression to enter the orbit and fall into the 

 orbital venous sinus. The ectethmoidal depression of this fish 

 thus corresponds, in general position and in its relations to the 

 nasal vein, to the ectethmoidal chamber of my desci'iptions of 

 CJilamydoselachiis (Allis, 1913), that chamber being, in certain 

 of the Selachii, an open fossa which may, as in HejJtanchus, be in 

 direct communication with the cranial cavity through the basal 

 communicating canal of Gegenbauer's descriptions. There is, in 

 Chimcera, no indication of this latter canal, but it is perhaps 

 woi'thy of note that in Lejndosiren there is, in just this region, 

 a median perforation of the basis cranii (Bridge, 1898). 



At the postero-mesial corner of the ectethmoidal depression of 

 Chimcera, near the summit of the ridge that there bounds the 

 depression, there is a groove which leads postero-mesially to a 

 canal which travei-ses tiie cartilage of the basis cranii and ti-ans- 

 mits the efierent pseudobranchial, or so-called anterior carotid 

 artery, this canal opening on the dorsal surface of the cartilage 



