William Patten. 



III. Descriptive Part. 



The General Shape of the Cephalic Shield of Tremataspis is so well shown by the figur- 

 es, that it requires but little description. The model from which the figures were made, 

 was constructed in wax ofter some eight or ten specimens of T. Schmidti, some belonging 

 to the Petersburg Academy and others to the Museum of Dartmouth College. In this species 

 the head is much flattened, and quite different from the strongly convex dorsal and ventral 

 surfaces of certain undescribed specimens that probably represent a new species. When seen 

 from above, the anterior end of the shield is somewhat pointed, with a slight angle, or 

 shoulder, near each lateral opening. The marginal serrations begin a little in front of the 

 posterior pair of lateral openings, gradually shifting their position at the posterior end of 

 the head on to the dorsal surface of the cornua. In the region of the four tubercles, there is 

 a gentle swelling of the dorsal shield, and in front of it a more pronounced one on which are 

 situated the median eyes and the olfactory organs. 



The dorsal shield is flattened on the sides, and shelves off at a sharp angle on the 

 margins, suggesting in cross sections at this point the cephalic shield of Limulus 

 (PL I. fig. 3.). 



The whole anterior end of the head bends slightly downwards and gradually merges 

 into the ventral shield. 



The Margins of the Oral Region: On the under side of the head is a large space that 

 we shall speak of as the oral region. It is normally occupied by numerous bony plates, but 

 in most specimens the plates have disappeared. The opening is surrounded in front and on 

 both sides by a narrow irregular rim, formed by the turned over margins of the dorsal 

 shield. The rim, which forms an extremely important morphological feature, is very fragile 

 and great care is required to expose it without destroying its contours. I have successfully 

 exposed various parts of it in several instances, and have obtained important data which 

 differ materially form those of.Rohou, as shown in his restoration of this region. 



Beginning at the anterior end, PI, I. fig. 1 . and PI. II. fig. 8. we find a low, beak-like flexure of 

 the dorsal shield, culminating on either side in a polished, rounded tooth-like projection. In the 



