On a Rf'pfih'itn Tooth with two Roofs. 227 



EXPLANATION OF PLATE IV. 



Fig. 1. Body of animal from above, somewhat flattened. 



Fig. 2. Ditto from the side. 



Fig. 3. Head viewed from the left front. «.«. = first antennre. 



Fig. 4. Last segment of abdomen from below. 



Fig. 5. Abdomen from below, showing the opercular plates. 



tig. 6. One of the legs. 



XXXVII. — On a Reptilian Tooth with two Roots. 

 Bj H. G. Seeley, F.R.S. 



The (livi.^ion of the root of a mamaialian tooth into two or 

 more portions has been regarded as a convenient means of 

 predicating mammalian organization for the animal in which 

 this condition is found, notwithstanding the circumstance 

 that in diverse groups of mammals the rojt is not divided in 

 any of the teeth, and that in mammals the division is absent 

 from the incisors and almost all canines. 



Professor Marsh, in 1890, figured, in the '■ American 

 Journal of Science,' teeth of the animal which he named 

 Triceratops, in which two roots certainly occur, but placed 

 transversely, as sometimes happens among the wider posterior 

 molar teeth of mammals. It is not improbable that this 

 division, as American palaeontologists have suggested, is 

 apparent rather than real, and has been produced by absorp- 

 tion of the tooth in wear, by the successional tooth rising 

 beneath it, since the form of the excavation between the roots 

 exactly corresponds to the form of the crown. In any case, 

 the condition in this American fossil, by whatever name the 

 genus may be eventually known, was unparalleled among 

 Keptilia, though in a few mammals witii two roots to a poste- 

 rior molar tooth those roots are arranged transversely. 



In 1854 (Quart. Joarn. Geol. Soc. vol. x. p. 420) the late 

 Sir Richard Owen figured a Purbeck fossil from bed K. 93 

 in Austen's guide, under the name Nuthetes destructor. It 

 was then described asa pleurodont lizard allied to monitors of the 

 genus Varanus ; and tigs. 2 d and e {I. c.) are representations 

 of teeth in the jaw which have the aspect of possessing two 

 roots arranged in the antero-posterior direction. Tliis con- 

 dition is further evidenced by tiie enlargement of the tooth d 

 given in fig. 4, though no word occurs in the text referring to 

 the structure ; so that it is probably only a pit or groove at the 

 base of the crown. With these fragments of jaw the author 



