Diastema in some Fossil Reptiles. 377 



so various as certain marsupials, chevrotains; horse, rhino- 

 ceros, pigs, rodents, so that no special importance can be 

 claimed for the diastema in morphology or classification. 

 Among some Mammalia there is evidence that the diastema 

 is a consequence of shedding of deciduous teeth, as well as of 

 the atrophy and suppression of teeth. There is reason for 

 supposing that these fossil reptiles had a normal dental 

 succession, in which a first or milk-series of teeth was 

 followed by a permanent series ; but there is no reason to 

 believe that the reptilian teeth were pushed out and shed in 

 quite the mammalian manner. The process of absorption ot 

 old teeth was carried much further in reptiles, and though no 

 evidence has been seen of successional molar teeth among 

 Theriodonts, the canines constantly have upon the roots 

 unabsorbed portions of the teeth which preceded them, situate 

 anteriorly in the mandible and posteriorly in the skull. 

 This mode of succession may account for the occasional 

 duplication of canine teeth, such as is found in Cynognathus 

 leptorhinus, the one tooth being a milk-tooth, and the other 

 permanent. There being no evidence of pushing out of the 

 first set of teeth, which correspond to milk-teeth which are 

 not replaced in mammals by permanent teeth, it follows that 

 they can only disappear by a condition of weakness, feeble- 

 ness as distinct from disease, which ensures inability to 

 persist so well as the permanent molar teeth. The reptilian 

 diastema therefore appears to be the portion of the alveolar 

 border from which the crowns of teeth of the " milk-series " 

 have crumbled away in the mature animal. Even with this 

 suggested explanation there remains a short interval in the 

 jaw without teeth behind the canine tooth which has to be 

 accounted for. The teeth of the molar series in these fossil 

 reptiles as they extend forward gradually diminish in size, 

 exhibiting a species of atrophy ; and it may be that nutrition 

 fails as work diminishes, and teeth are never developed. 

 Hence the reptilian diastema includes two elements — an 

 anterior part, which originates in the mammalian way ; and a 

 posterior part, which illustrates the reptilian type of a false 

 diastema, which may be regarded as a condition antecedent 

 to the type which becomes developed as a true diastema in 

 mammals. 



In 1895, after discussing reptilian characteristics of the 

 skull of Tritylodon longcevus (Owen), I gave a figure (Phil. 

 Trans. Royal Society, 1895 B, p. 1028, fig. 4) of the anterior 

 extremity of the right ramus of the mandible of a Theriodont 

 reptile as illustrating the kind of mandible which that genus 

 might possess, and as indicating the possibility that the 



