62 MOSASAURUS. 
tooth last described. ‘The crown when perfect has measured less than three-fourths 
of an inch in length, is elliptical in transverse section, and measures at base five 
lines and three-quarters antero-posteriorly, and four lines and a half transversely. 
Its single posterior carinated ridge is minutely denticulated as in the large teeth, and 
the surfaces are devoid of planes. 
The fang, independent of the alveolar fragment with which it is coossified, 
measures about an inch and a half long, and has at its inner side posteriorly a deep 
excavation for a successional tooth, as seen in Fig. 5, a. 
15. A perfect tooth, coossified with a fragment of the jaw, from Monmouth 
County, New Jersey, loaned by William Cornell, through Prof. Cook. The speci- 
men was received after the present memoir and its accompanying plates were 
nearly completed. It closely resembles the eighth specimen of the series above 
described. The crown is unworn, is twenty lines long, and is elliptical in transverse 
section. Its base is one inch in diameter antero-posteriorly, and nine lines and a 
half transversely. It possesses a single ridge, situated along its posterior or concave 
border; and the surfaces are smooth, except that the outer one presents a feeble 
disposition to subdivision into four planes. The lower of the accompanying outlines, 
No. 17, represents a section from near the base. The fang is three inches long. 
16, Two teeth, from Monmouth County, New Jersey, presented to the Academy 
of Natural Sciences by Dr. J. H. Slack. One of the specimens resembles the two 
large ones last described. The crown has the apex broken off, but is otherwise 
perfect. Its transverse section is elliptical, and measures at base antero-posteriorly 
thirteen lines; transversely ten lines and a half. It possesses a single carina, 
situated posteriorly, and the surfaces are totally devoid of planes. The accompa- 
nying right hand outline, with one point, No. 18, represents a section near the base. 
The fang is about three inches long, and exhibits on its inner side near the centre 
a slight excavation, five lines long, as the commencement of a cavity for a successor. 
The canal communicating with the pulp cavity through the 
fang is open. 
The second specimen, represented in Fig. 7, Plate IX, is 
nearly perfect, and measures about four inches and a half long. 
It corresponds in all its anatomical characters with the teeth 
described by Prof. Owen as characteristic of a distinct genus, 
to which he has given the name of Letodon. 
The crown is twenty-one lines long, elliptical in transverse section, as represented 
in the accompanying left hand outline, with two points, No. 18, and measures at 
base antero-posteriorly thirteen lines; transversely eleven lines. Minutely denticu- 
lated ridges divide it in the usual manner into inner and outer surfaces of nearly 
equal extent and convexity and totally destitute of subdivisional planes. 
The fang is straight, and presents no trace of having been coossified with its 
alveolus, as is also the case with that of the preceding specimen. It further exhibits 
no trace of a cavity for a successional tooth. 
No. 18. 
17. The shed crown of a tooth, from St. Georges, New Castle County, Delaware, 
contained in the museum of the Academy of Natural Sciences. — It is represented 
in Fig. 11, Plate IX, and resembles the corresponding part of the tooth just 
