3d. K. LYDEKKER OX THE CEIACEA 0E THE SrEEOLE CRiG. 



13 



improbable that a small tympanic in the latter collection may also 

 belong to this spe3ies. A tympanic from the Red Crag, in the Museum 

 of Practical Geobgy (represented by a cast in the British Museum), 

 indicates the occurrence of Herpetocetus scaldieasis, Yan Beneden, in 

 this country, and Xo. 2316 * in the Museum of the College of Sur- 

 geons is a second example. The tympanic of this genus (which exhibits 

 some affinity in the structure of the mandible with the Physeteridat) 

 is readily recognized by its egg-like shape, the small and sharply 

 defmed involucrum, and the fHling-up of the anterior portion of the 

 cavity by osseous matter. 



Phy set er idee. — In the Physeteridxz the periotict (which, as in the 

 other families of the Odontoceti, is not anchylosed to the tympanic) 

 articulates anteriorly by a smooth facet (or, PI. II. fig. 6) with the 

 tympanic, and posteriorly is broad and has a distinct median longi- 

 tudinal ridge (b) on the same face for articulation with the free 

 border of the latter bone. The genus Eucetus,. Du Bus, which 

 appears to be allied in dental characters to Physeter, is represented 



Fig. 2. — Eucetus amblyodon, Du Bus. The left periotic ; from the 

 Red, Crag. Two thirds nat. size. British Museum (Xo. 

 27854). Letters as in Plate II. 



i 



3 



in nearly all Crag collections by many teeth, which belong to the 

 type species E. amblyodon ; the cement is of great thickness, the 

 dentine-core fusiform, and the osteodentin e nodular. I provisionally 

 refer to this species i a large left periotic in the British Museum 

 (woodcut, fig. 2), which in the partial production of its posterior 

 extremity more nearly resembles the periotic of Hyperoodon than 



* Entered in the Catalogue under the head of Balcena definita. 



t While the tympanic is the most characteristic bone in the BaZ&nidce, the 

 periotic (which is more commonly preserved in the fossil condition) is the one 

 affording the best generic characters in the ~Phygeterid.ee and Delphmfdee. 



X On account of its large size and the circumstance that teeth of Eucetus are 

 much commoner than those of Bcdcenodon. 



