BRITISH FOSSIL CETACEA 



'I'lic claims of ZipJiiiis Layardi, for example, to the generic distinction conferred on it, 

 under the name Dolichodon, appear to rest on modifications of the mandibnlar pair of 

 teeth, which are described as " very long, produced, arched, and truncated, with a conical 

 process in front." * 



I have figured the obtusely rounded extremity of the left of these teeth of the natural 

 size, in PI. I, fig. 5. The first calcified part of the tooth-pulp, which still retains a thin 

 coat of enamel, must have appeared as the ' germ ' of the tooth in the young Ziphius, 

 and Avas probably one of a series in the alveolar groove. Its formation in the present tooth 

 has been followed by a rapid growth of matrix in the antero-posterior direction, which 

 became calcified without enamel, and was progressively added to from below until, finally, 

 a tooth of 1 foot in length, \\ inch to 2 inches in fore-and-aft breadth, and from 3 lines 

 to 6 lines in thickness, was completed, and most probably, as a sexual character. In the 

 present specimen about three inches of the base of the tooth is obliquely inserted into a 

 cleft of the alveolar border of the mandible (c, c, fig. 4, PI. I) ; the base of the tooth 

 terminates in a solid jagged border, showing exhaustion of the pulp and cessation of 

 further growth, but abrasion of the fore part of the base of the exposed crown of the 

 tooth has exposed a shallow remnant of the pulp-cavity. This abrasion seems due, as 

 Dr. Gray observes, to "friction of the upper jaw."^ The dense rostrum rested in part 

 thereon when the mouth is closed, and the teeth curved backward and inward so as to 

 embrace the rostrum, as shown in Mr. Trimen's drawing, ^ reproduced by Dr. Gray in his 

 ' Catalogue ' above cited. 



Many teeth of Cetacea from the Red Crag have reached me slioAving abrasion of 

 the dentinal part or body of the tooth below an apical or coronal part, on which a thin 

 layer of enamel is traceable. The analogy of the structure of the mandibular teeth of 

 Ziphius Layardi supports a reference of such Crag fossils to the same genus, and I 

 have observed nothing in the anterior laniariform teeth of Zeuglodon (or Sqitalodon) to 

 affect such view. 



It is probable that the mandibular teeth, developed as in Zipldiis Layardi, are peculiar 

 to the male, but the proof is wanting. It is more probable that they exhibit an abnormal 

 direction and state of growth in the specimen figured in PI. I. In any case I cannot regard 

 such modifications of form or size of the pair of laniariform mandibular teeth, character- 

 istic of Ziphius, Cuv., as grounds for further splitting up this well-marked and intelligibly 

 defined genus. 



Petkorhyncuus, Gray. — It will be seen that I have availed myself of the various degrees 



1 Gray, ' Catalogue of Seals and Wliales in the British Museum,' 8vo, 18GG, p. 3.")3. 



- lb., p. 3.55. Dr. Gray's expression, "The edges of the front lower teeth are absorbed or worn 

 away," &c., would leave tlie inference that there were otber teeth behind, which is not the case; he doubt- 

 less meant to say " the front edge of the lower teeth," but it is a small part only of that edge which is so 

 affected. 



3 'I'roc. Zool. Soc.,' April 11, 18G,">, p. 358, fig. e. 



