86 IOWA GEOLOGICAL SURVEY 



and certain conjectures are hazarded by him as to the course of 

 the olfactory nerves, which will be referred to presently. The 

 median plate of the ocular opening is nearly perforated by a 

 deep pineal foramen, similar to that occurring in Pterichthys, 

 and often indicated by a low tubercle on the external surface. 

 Two other pits, shallower than the first, and without any external 

 indications of their presence, are symmetrically placed behind it 

 on the under side of the small postmedian plate. The unpaired 

 pit is in all probability to be regarded as the impression of the 

 pineal body, but it is difficult to imagine what may have been 

 the nature or function of the posterior pair. Analogy with all 

 other vertebrates tends to discredit Professor Patten's sug- 

 gestion that these three' pits are all of the same nature and to- 

 gether indicate the presence of a triocular median eye.* 



A matter of absorbing interest is the arrangement of the mouth 

 parts in this species, now very satisfactorily known. Beneath 

 the headshield at its front extremity (text -fig. 13) there is 

 observed a pair of thin, concave plates of bone with free median 

 and posterior margins, the latter being sharply bevelled and 

 serrated. These plates are quite similar to those called the 

 maxillae in Pterichthys (text-fig. 10 m%, page 77), but are less 

 extensively in contact mesially, and are notched at the antero- 

 external instead of at the postero-external angles. The greater 

 part of the exposed surface of each plate is feebly rugose and 

 marked by a sharply bent sensory canal, all appearances sug- 

 gesting that they are purely dermal ossifications, and therefore 

 of different origin and nature from the gill-arch jaws of higher 

 vertebrates. The plates in question may be conventionally in- 

 terpreted as maxillae, and are in fact so designated by Traquair, 

 though Patten insists that they are mandibular. In the ma- 

 jority of specimens these so-called maxillary plates, and also 

 the smaller, sigmoidal pair of mandibular elements, using both 

 terms in the Traquairian sense, are more or less displaced; but 

 this purely accidental feature furnishes no reason for supposing 

 that the two pairs did not work directly against each other, or 

 that the right and left halves of either pair were capable of 

 independent motion, as in Arthropods. Finally it may be re- 



* Patten, W., New Facts concerning Bothriolepis. Biol. Bull. 1904, 7, p. 121. 



