60 Prof. R. Owen on rare Extinct Vertebrates. 



teeth, attributed to the genus Leiodon and exemplified by 

 transverse sections in fig. 15, PI. VIII., has been found con- 

 stant and characteristic of those teeth in the species of the 

 genus noted as from the various localities in Leidy's ' Mo- 

 nograph on the Cretaceous lieptiles of the United States,' 

 and represented in pis. ix.-xi., and in the woodcuts of 

 pp. 58-69. The subject of Leidy's cut no. 18, and of fig. 7, 

 pi. ix., from Monmouth County, New Jersey, is noted as 

 corresponding in all its anatomical characters with those of 

 Leiodon (p. 62). 



The Mosasauroid or family characters of the teeth are 

 strictly retained. The crown, composed of hard dentine with 

 a thin coat of enamel, is supported by, and as it were wedged 

 into a basis (fang or root) of the modified osteine called 

 " cement." This is much thicker than the crown, and pre- 

 sents a rounded or full elliptical transverse section, whatever 

 be the shape of the crown (compare the outline a (root) with 

 b (crown) of a tooth of Leiodon , fig. 15, PI. VIII.). 



The root, when first formed, is implanted in a distinct 

 socket, and the " thecodont " type of dentition is manifested. 

 But this is a transitory condition ; the cement becoming con- 

 fluent with the bone of the socket, and, partially rising above 

 the alveolar border of the jaw, the tooth then exhibits the 

 " acrodont" type. But a much larger proportion of the orginal 

 and independent tooth can be traced in the substance of the jaw 

 than is the case in the existing acrodont Lacertilia. The pulp- 

 cavity remains in the basal half of the crown, and descends a 

 short way into the cemental fang, where it is closed by a 

 coarser and more vascular modification of the cement. The 

 vertical or longitudinal extent of the enamelled crown at its 

 outer and exposed surface is about one third that of the fang. 



The teeth are displaced and succeeded by others, many 

 times during the life of the individual, as in modern Lacertilia. 

 At least I have met with no specimens in which a reserve 

 cavity with a more or less advanced successional tootli has 

 not existed at the postero-internal side of the implanted base 

 of the functional tooth*. The germ of a third generation in 

 the form of the enamelled apex of the crown may be detected 

 within the pulp-cavity of the second, which is in course of 

 succeeding and displacing the first or fully formed tooth |. 



The nearest resemblance to the Mosasauroid type of tooth 

 is now presented by certain Cetacea, as, e. (/., the Cachalots \, 

 Platanists§, and more especially by the Ziphioids, in which 



* ' Odontography,' p. 251), pi. 72. lig. 2. 



t See Leidy, id .supra, pi. xi. lig. 4, d. 



X ' Odontography,' pi. 89. fig. 2. § lb. pi. 87 a. 



