126 IOWA GEOLOGICAL SURVEY 



Little need be added by way of supplementing the above de- 

 scription. The total length of the dental plates rarely exceeds 

 5 or 6 cm, though in one specimen in the Cambridge Museum it 

 amounts to 8 cm. There is much similarity between upper and 

 lower dental plates, barring the not uncommon circumstance 

 that the symphysial margin of the lower is produced into a long 

 and slender descending spiniform process, and the sectorial edge 

 of the upper is somewhat less arched or "excavated" (whence 

 the specific title), than in the lower dental plates. As shown 

 by marks of wear, the tips of the lower dental plates closed out- 

 side and slightly behind those of the upper, after the same man- 

 ner as in Ptyctodus. An extensive series of specimens, collected 

 many years ago by Mr. Orestes H. St. John from the Cedar 

 Valley limestone near Waterloo and Waverly, Iowa, is now pre- 

 served in the Museum of Comparative Zoology at Harvard 

 College. 



The dental plates of this species are accompanied both in the 

 Cedar Valley limestone, and in the Hamilton of Milwaukee by 

 numerous detached scale-like dermal ossifications, somewhat sug- 

 gestive of those of Myriacanthus,* but presenting much closer 

 affinities with the contemporary "genus" Acanthaspis. As 

 indicated by their almost lamellar thinness and tuberculate orna- 

 mentation, these plates were without doubt externally situated, 

 possibly in the vicinity of the pectoral fin. An average-sized 

 plate of this kind, from the Hamilton of Milwaukee, is shown in 

 Plate II, Fig. 18. Plates exhibiting the same form, which is 

 constant, some of equal size and others one-quarter to one-third 

 larger, but all extremely tenuous, occur in the Cedar Valley 

 limestone. They are clearly of identical nature with the ele- 

 ment designated as "cleithrum" by Jaekel, found in natural 

 association with the dentition of his so-called Rhamphodus 

 tetrodon, and interesting for the enlightenment it furnishes in 

 regard to the so-called Acanthaspis of Newberry. One or two 

 examples of the smaller structure interpreted by Jaekel as 

 "clavicula" are also known from the "Waterloo locality. These 

 bodies will be referred to more particularly under the head of 



* A good example is figured by Smith Woodward in his Cat. Foss. Fishes Brit. 

 Mus. 1891, pt. 2, pi. 2, fig. la. 



