CHIALEROIDEI. 55 



is triturated by wear. In the surviving family of Chimseridae 

 these dental plates are much thickened, while the hinder upper 

 pair (" palatines ") are both closely apposed in the median line 

 and considerably extended backwards. The Jurassic families 

 of Squaloraiidae and Myriacanthidse, however, exhibit a more 

 primitive arrangement. Their dental plates are thin, pre- 

 senting considerable superficial resemblance to those of certain 

 Cochliodont Elasmobranchs (see p. 42). The hinder upper 

 plates are not closely apposed in the median line and the 

 tritoral areas have not any very definite limits or disposition. 

 The Cochliodont plates being proved to result from the fusion 

 of normal crushing teeth, it is thus easy to conceive how the 

 Chimaeroid plates may have been developed in a similar manner, 

 the evolution having only proceeded further so that growth 

 is not restricted to the inner border, as in Cochliodonts, but 

 extends to the whole of the attached surface. 



The dental plates named Ptyctodus, from the Devonian of 

 Russia and North America, are, as already remarked, essentially 

 similar to those of modern Chimaeroids, but there are no re- 

 presentatives of the vomerine pair. The tritors, two in number 

 in the typical species, are well differentiated, consisting of hard, 

 punctate, superimposed laminae, arranged obliquely to the 

 functional surface. The contemporaneous teeth known as 

 Rhynchodus and Palceomylus, however, exhibit more indefinite 

 tritoral areas. The symphysial facette is always distinct. 



Spines which may be compared with those of Chimaeroids 

 are also known from Devonian and Carboniferous formations, 

 and Harpacanthus may perhaps be cited as an example of a 

 head-spine. No Chimseroid skeletons, however, have hitherto 

 been satisfactorily determined from Palaeozoic rocks. 



The family of Squaloraiidae, known only by the genus 

 Squaloraja from the English Lias, has already been mentioned 

 as exhibiting a comparatively non-specialized dentition. The 

 whole of the skeleton, except that of the median fins, is now 

 tolerably well known. 



Squaloraja (fig. 44). The body is depressed but elongated, and the 

 head is produced into a flat rostnim without lateral teeth. The tail 

 gradually tapers to a point, and the median fins are not preserved in 



