78 IOWA GEOLOGICAL SURVEY 



ture. The two pectoral limbs consist also of plates similarly 

 united, and are internally hollow so far as their remains in the 

 stone are concerned. 



The headshield (test-fig. 9) is semielliptical in shape, rounded 

 in front and truncated behind, where it joins the system of 

 body plates. In the center it shows a transverse aperture, the 

 median opening or orbit, slightly contracted in the middle and 

 expanded at each of its rounded sides. This opening is in per- 

 fect specimens filled up by at least three other plates, which, 

 being loose, are usually lost. Of these, one is in the center, 

 quadrate in shape, but with concave outer margins, and may 

 be called the median or pineal plate (m), as it shows on the in- 

 ternal aspect a shallow rounded pit, pointed out by Dr. Smith 

 Woodward as probably the impression of the pineal body. This 

 is, by its outer concave margins, in contact on each side with a 

 rounded convex ocular plate (o), indicating certainly the posi- 

 tion of the eye, but whether or not due to an ossification in the 

 sclerotic is doubtful. [In Bothriolepis the orbits have two 

 sclerotic plates each, according to Professor Patten.] The 

 nuchal region is occupied by a large plate, the median occipital 

 (m. occ), shaped something like the conventional royal "crown", 

 but without the pinnacle in the center. In front of this and im- 

 mediately behind the median opening is a smaller plate, the post- 

 median (pt. m.) ; while between the anterior margin of that 

 opening and the front of the cranial shield is one of larger size 

 and somewhat quadrate shape, the premedian (p. m.). Two 

 large pieces, the lateral plates (I), one on each side, bound the 

 opening laterally, and also extend to the front of the shield. Be- 

 hind these, and forming part of the hinder margin of the buckler 

 external to the median occipital, are two other paired plates, 

 the lateral occipital (I. occ.) and the angular (ag). 



The upper and lateral aspect of the cranial shield is now com- 

 pleted by a plate on each side, which is only loosely articulated 

 in Asterolepis and Pterichthys, though formerly sutured in 

 Bothriolepis. This is the extra-lateral (e. I.) or opercular plate, 

 as it has also been called by some writers. 



On the lower aspect of the head and close behind the anterior 

 margin of the shield are two transversely oblong plates (mx), 



