MUSEUM OF COMrAUATIVE ZOOLOGY. 217 



ventricle (vnt.^) into the epiphysis. Intimately connected with the dis- 

 tal end of tlie portion of the epiphysis tluis far described is found a 

 vesicle (epk. vs.), the thick walls of which are composed of columnar 

 epithelium, and thus differ markedly from the folded epithelium of the 

 anterior wall previously described. This vesicle is much flattened 

 antero-posteriorly, its longest axis lying very nearly in the axis of the 

 cylinder to which the epiphysis as a whole has been compared. That 

 the structure here described is a separate vesicle, and that its cavity 

 is not continuous with the cavity already described as a continuation 

 of the third ventricle, admit of easy and satisfactory demonstration, 

 not only in this particular instance, but also in all other individuals 

 both of this species and of P. coronata of wiiich sections have been 

 made. In passing through the entire sei'ies of sections, it is easily seen 

 not only that the two cavities nowhere approach more nearly to conflu- 

 ence than in the one represented in the figure, but also that the walls 

 of the vesicle and those of the more proximal part of the epiphysis with 

 which they are in relation are clearly distinct. The sejjarateness of 

 these two structures will appear more clearly when we come to consider 

 the same parts in P. coronata. Passing upward and forward from the 

 distal end of this vesicle is to be seen a bundle of connective-tissue 

 fibres which becomes blended with the string of connective tissue already 

 described as runnitig from the apex of the cone to the region of the parietal 

 vesicle. There is no indication that the epithelial wall of the epiphysial 

 vesicle, as it may be called, passes into this string. 



Covering the whole postero-dorsal convex side of the portion of the 

 epiphysis thus far described, and even extending considerably beyond 

 its distal extremity, is an immense blood sinus fully distended with 

 blood corpuscles (Fig. 9, sn. sng., and Fig. 8, cp. sng.). 



Phrynosoma coronata. 



1. General Description. — Figure 2 (Plate I.) represents a transverse 

 section of the dorsal wall of the head, passing through the middle of the 

 parietal eye of P. coronata. The description of the external appearance 

 and of the vesicle and its surrounding structures given for P. Douglassii 

 requires modification in only a few points to become applicable to this 

 species. The depression mentioned as existing on the internal surface of 

 the wall of the brain-case immediately under the vesicle in P. Doug- 

 lassii becomes in this species a deep pit. To correspond with this 

 pit the external surface of the wall immediatel}' over the vesicle forms 



