198 PALEONTOLOGY OF ILLINOIS. 



species is produced by the wrinkling of the enamel or external layer, 

 and which apparently has no other relation to the medullary tubes 

 than to rudely define them in irregular and transverse or longi- 

 tudinal rows, the punctae rarely confluent, and the rugose appear- 

 ance becoming obsolete or more or less obscured over the more 

 exposed parts of the triturating surface. The impression also pre- 

 vails that the tendency to rugosity of the coronal surface increases 

 with age, since this appearance so far as observed seems to be most 

 prevalent and conspicuous in large individuals belonging to the 

 series which have received several accessions, the innermost indi- 

 viduals of which have suffered little from the abrading effects of 

 trituration while in use ; but it is not an essential character, as 

 some species evidently always remained quite smooth in their cor- 

 onal areas. The inferior surface is plane, in a general way con- 

 forming to that of the crown, and even possessing distinctive 

 characteristics as applied to species ; it shows in the perfect state 

 a rather dense thin layer, perhaps in degree rather than structually 

 differing from the more cellulose middle layer composing the bulk 

 of the base, and usually marked by more or less distinct longitudi- 

 nal grooves, or smooth, and faintly keeled nearest the inner articular 

 border. 



In drawing up the foregoing diagnostic account of Psammodus, the 

 authors have been actuated by the desire to place before the student 

 of palseichthyology such data as they themselves possessed relating 

 to the genus, and which they owe largely to the kind offices of their 

 colaborators. It will have been observed that the genus embraces 

 a variety of forms, which, while their generic identity is unques- 

 tioned, offer so diverse appearances as, in the absence of other than 

 detached and fragmentary remains, to greatly complicate the de- 

 termination of the relative position the forms occupied upon the 

 jaws. Their congeneric identity is proven by identity of outline and 

 contour and superficial structure common alike to each and all of 

 the forms, and these latter resolve themselves into certain well- 

 defined groups, so that specific distinctions may be recognized in 

 unique examples pertaining to ons or the other form. But in the 

 attempt to associate these forms under definite, specific combina- 

 tions, the observer is necessarily compelled to rely, to some extent 

 at least, on other than superficial resemblances, as, for example, 

 the association of the individuals in the horizons whence they were 

 derived. The process is often further complicated by the accidents 

 of collecting, for it not unfrequently happens that a series of teeth 



