CRETACEOUS CHIM.^.ROIDS. 1 47 



CRETACEOUS CHIM^ROIDS. 



Chimseroids, it may finally be remarked, were at their maximum evolutional 

 development during the Cretaceous period; they were then represented by the 

 greatest number of genera and of species (about 50 species), a result which maj' 

 well have proceeded from the acquisition in their line of some new "expression 

 points"; such, for example, may have been the apposition of meckelian and chisel- 

 shaped subnasal "vomerine" plates, which must have added vastly to the effective- 

 ness of this type of dentition; also the greater development of the clasping organs; 

 also, perhaps, deep-water adaptations which enabled these forms to enter a new and 

 rich field for development. Certain it is that these Cretaceous Chimseroids were 

 of a distinctly modern pattern, and one of them is even assigned to the recent 

 genus Callorhynchus. 



The details of the evolution of recent genera from their Cretaceous ancestors 

 are unfortunately meager. Dental plates and spines are practically the only 

 evidence at hand for comparison. If, however, we limit our studies to dental 

 characters, we can at least conclude that their evolution has been in the line of 

 producing tritors either in marginal or in centralized arrangement. In Ischyodus, 

 for example (fig. 124), it will be seen that some of the tritoral areas of the palatines 

 and meckelian plates are becoming localized near the median line. In Edaphodon 

 (fig. 123) the tritoral areas of even the vomerines are more nearly median; indeed 

 the only conspicuous appearance of marginal tritors occurs at the tip of the meck- 

 elian plates. Elasmodus (fig. 122) indicates an interesting combination, since it 

 has developed both the marginal and the median series of tritors. It has thus a 

 dentition of a generalized character, and one is not surprised to find that it passes 

 over from the Cretaceous into the Eocene. In fact, it differs little from the denti- 

 tion of the recent Harriotta. On the other hand, Elasmodectes (judging from its 

 meckelian plates, which alone are accurately known) represents a form which is 

 specializing in the direction of marginal tritors; they are numerous, continuous in 

 arrangement, and minute in size, and altogether the plates were probably beak- 

 like in function. This type of dentition appears at first sight too specialized to 

 have long survived. Nevertheless, granting a continued reduction of these minute 

 marginal tritors, and more flattened and beak-like arrangement of the plates, a 

 descendant of Elasmodectes might well be represented in the recent Rhinochimsera. 



As far, therefore, as a study of the dental plates alone is concerned, one might 

 conclude reasonably that the recent genera were descended from Mesozoic forms 

 in somewhat the following way: Callorhynchus from an ancestor closely related to 

 Edaphodon, Chinicera from Ischyodus, Harriotta from Elasmodus, and Rhinochi- 

 msera from Elasmodectes. Such genera, for example, as the Cretaceous Lepto- 

 mylus and the Miocene Mylognathus are apparently already too specialized to have 

 represented the ancestral condition of the living forms. There can be no question 

 that, with the exception of the three genera first named, the Mesozoic, Tertiary and 

 recent Chimseroids are a single and homogeneous stock. They have none of the 



