178 Shinier — Lower-Middle Cambrian Transition Fauna. 



Dr. G. F. Matthew, who kindly examined the photograph 

 of the specimen, suggests that it may be a group of pleura 

 belonging to one of the spinous trilobites. This would explain 

 its curved character, indicated by the lack of parallelism 

 between the transverse grooves. 



This specimen is in the collection of the Boston Society of 

 Natural History; catalogue number 13, 341. 



Agraulos quadrangular is (Whitfield). 



This species is exceedingly abundant but most of the indi- 

 viduals differ from the normal mature form of the Hayward 

 Creek Middle Cambrian. In transverse view the glabella is 

 higher and more arched ; the longitudinal contour is of uniform 

 elevation in the posterior two-thirds and descends abruptly in 

 the anterior one-third, but as its height is greater than that of 

 the normal species so is its descent more conspicuous. The 

 anterior edge of the cephalon is truncate even in those forms 

 showing no compression, while in the normal species it is 

 rounded.* 



The sides of the glabella are sub-parallel and do not taper 

 strongly forward as is usually the case with the normal form. 

 The spine is about half the length of the cephalon exclusive of 

 the occipital ring, while in the normal form it is about one- 

 third. 



The very small (young) and very large (old) forms of A. 

 quadrangular is of Hayward Creek beds are quite similar to this 

 variety. It thus seems that this variety represents an earlier 

 (younger) form of the normal (mature) species and that the large 

 forms of the latter were merely gerontic specimens showing a 

 return in shape to the characters of youth. 



Pbychoparia rogersi Walcott. 



One specimen, somewhat crushed laterally, represents the 

 anterior and most of the central portions of the cephalon. The 

 anterior head fold is well shown and the glabellar outline. 



Another specimen consisting of only the most anterior por- 

 tion of the glabella and cephalon shows this anterior fold more 

 faintly. 



Paradoxides harlani Green. 



There appears to be no difference between our many repre- 

 sentatives of this species and the normal mid-Cambrian forms 

 from Hayward Creek. One individual attained an enormous 

 size, measuring over three inches across the wider anterior 

 portion of the glabella. Several hypostomes were found, one 

 two inches wide and attached to the frontal doublure of the 

 head. Separate pygidia were also quite abundant. 



Massachusetts Institute of Technology, Geological Department. 

 *Bull. Am. Mus. Nat. Hist., i, 147. 



