. Bulletin. Natural history. RUSSELL: AMERICAN MOSASAURS 135 those referred to M. lemonnieri from the Afaestrichtian of Belgium (see Dollo, 1894). SDSM 452 has a vertebral formula of 7 cervicals, 38 dorsals, 8 pygals and about 75 chevron-bearing caudals, as compared with 7 cervicals, 39 dorsals, 12 pygals and about 76 chevron-bearing caudals for M. lemonnieri. In the dorsal series there are 17 tlioracics and 21 lumbars as compared with 18 thoracics and 21 lumbars in M. lemonnieri. The zygopophyses become functionless on the first chevron-bearing caudal of SDSM 452 and on the fourdi pvgal of M.
. Bulletin. Natural history. RUSSELL: AMERICAN MOSASAURS 135 those referred to M. lemonnieri from the Afaestrichtian of Belgium (see Dollo, 1894). SDSM 452 has a vertebral formula of 7 cervicals, 38 dorsals, 8 pygals and about 75 chevron-bearing caudals, as compared with 7 cervicals, 39 dorsals, 12 pygals and about 76 chevron-bearing caudals for M. lemonnieri. In the dorsal series there are 17 tlioracics and 21 lumbars as compared with 18 thoracics and 21 lumbars in M. lemonnieri. The zygopophyses become functionless on the first chevron-bearing caudal of SDSM 452 and on the fourdi pvgal of M. lemonnieri. In the caudal series the neural spine is vertical on postsacrals 42-44 in SDSM 452, and in M. lemonnieri it is vertical on post- sacrals 36-37, the longest neural spine occurring on postsacral 36 in both forms. The longest haemal spine occurs on postsacral 47 in SDSM 452 and on postsacral 46 in M. lemonnieri, and the transverse processes become rudimentary in the vicinity of the eighteenth caudal in both forms. It should be noted that marginal teeth in SDSM 452. Text-fig. 78. Tooth preserved in type dentary of Mosasaurus conodon (AMNH 1380, X s4)- and in the type of M. conodon are smoothly surfaced, but the teeth of YPM 1573 and the single tooth preserved in the dentary of USNM 18255 are narrowly faceted, as in M. lemonnieri. However Dollo (1894, p. 230) did not observe zygosphene-zygantrum articulations in the vertebral column of M. lemonnieri, which are functional in the anteriormost presacral series of M. conodon. Dollo lists 12 pygal vertebrae in M. lemonnieri, in SDSM 452 there are but eight. The posterior caudal centra of M. lemonnieri are verti- cally oval but in M. conodon they are horizontally oval. The zygosphene-zygantrum articulation may be variably developed within a species of Platecarpus and it is possible that they were either small and overlooked by Dollo in M. lemonnieri or were indeed absent. They are only present in a small portion of the column
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