130 



Mr. 0. Thomas. 



[May 16, 



point of development, as was then not unnaturally supposed, they 

 were merely commencing to undergo calcification within the tooth- 

 capsule, just as would have been the case with those of any other 

 young mammal. 



But in some ways the point that is of most importance in the 

 discovery of fully- developed Monotrematous teeth is the fact that for 

 the purpose of comparison with those of other mammals, a comparison 

 to which of late great attention has been directed, we have now 

 available perfect calcified teeth, of a size sufficient for inspection with 

 the naked eye, and very far superior to anything that figures compiled 

 from microscopic sections can possibly be. 



Such a comparison I would have willingly now made, but unfor- 

 tunately the most careful search* among other animals, fossil and 

 recent, mammalian and reptilian, fails to reveal any teeth quite 

 corresponding to those of Ornithorhynchus. But, nevertheless, their 

 study inclines one more and more to believe in the correctness of 

 Professor Cope'sf ingenious suggestion as to the Monotrematous, or, 

 as I should prefer to say, Prototherian, nature of the MesozoicJ 

 Multituberculata. These animals, long looked upon as Diprotodont 

 Marsupials, have of late been much studied in America,§ where large 

 numbers of them have been found. Many of them (e.g., Bolodon, 

 Allodon, Ptilodus, and, especially, the best known of all, Microlestes 

 and Plagiaulax) have molar teeth which are broad and low-crowned, 

 and which have a series of cusps running around their edges, so that 

 each tooth has two rows of cusps corresponding in a general way to 

 the cusps on one side and the crenulations on the other in the teeth 

 of Ornithorhynchus. A figure of one of the molar teeth of Microlestes 

 is given (fig. 5) to show how far it resembles those of the living 

 form. 



Still it must be insisted that the resemblance between the 

 Multituberculate- and the Ornithorhynchus-teeth is of the most 

 general character, and that the two are certainly widely separated 

 genetically, even if we do admit that they appear to possess a relation- 



* In this search I have had the advantage of the assistance of Mr. R. Lydekker 

 and Mr. Gr. A. Boulenger. 



f 'Amer. Nat., 5 vol. 22, 1888, p. 259. Professor Seeley's remarks in 1879 

 ('Quart. Journ. Greol. Soc.,' vol. 35, p. 456 et seq.) on the relationship to the 

 Monotremata presented by a Mesozoic humerus and femur assigned either to 

 JBhascolotheriwm or Amphitherium do not touch the question since neither of these 

 animals are Multituberculata, both belonging to the Polyprotodont division of the 

 Mesozoic mammalia. — May 17, 1889. 



X The Eocene Neoplagiaulax, Lemoine (Paris, ' Soc. Greol. Bull.,' vol. 77, p. 249), 

 .also belongs to this group, and has teeth that present a certain resemblance to those 

 of Ornithorliynchus. Compare Plate V, fig. 3, and YI, figs. 17-19, of that work 

 with the figures now given. 



§ See papers by Cope, Marsh, Osborn, Scott, and others- 



