40 The Ottawa Naturalist. [June-July 



of the cephalons. On the left are placed the generic character- 

 istics which connect P. saratogensis and P. armaius. In 

 working out of the indices, the length of the cephalon was meas- 

 ured from the middle of the neck furrow to the anterior extremity, 

 the length of the glabella being measured from the middle of 

 the neck furrow to the anterior edge. The index in each case 

 was derived by dividing the smaller by the larger measvirement 

 and multiplying the resultant by 1 ,000 to make it a whole num- 

 ber. It was hoped that by making careful measurements 

 (within 0.2 5 of a mm.) that the indices would afford valuable 

 criteria for the classification, but unfortunately the results do 

 not appear to be decisive, probably because of the lack of suf- 

 ficient working material. Nineteen specimens representing the 

 total available material of P. saratogensis, were measured and 

 their indices calculated. The average index proved to be 70 

 but a careful inspection of the individual indices showed this 

 figure to be tdo low. Fourteen of the indices range between 

 75 and 80; only two fall below 70, while two others are as high 

 as 85. The figures seem to show that further collecting would 

 raise the average considerably. It is also important to note that 

 measurements taken on P. saratogensis figured by Weller (6) as 

 representative specimens from New Jersey show an index of 80. 

 The writer has therefore made a conservative estimate of 75 

 as the index for P. saratogensis. So far as can be determined 

 at present the index does not vary between the forms A\dth 

 smooth and those with furrowed glabella. Eleven specimens 

 of A. ceticephalus were measured and their average index proves 

 to be 63. This average was shown to be practically identical 

 with that calculated from measurements made on Barrande's 

 figures. The writer was forced to calculate the index for P. 

 armatus from the original drawing by Billings. So far only one 

 cranidium of this species is known, and its high index (88) may 

 not be entirely indicative of the average for the species. All 

 the evidence seems to show that the indices of P. armatus and 

 P. saratogensis are very similar and dissimilar from that of 

 A. ceticephalus. In addition, P. saratogensis and P. armatus 

 have the following characteristics in common, which in turn are 

 dissimilar from those of A. ceticephahis: 



1. Greater convexity of the cephalon. 



2. Eyes close to the glabella. 



3. Cephalon never upturned at the anterior margin. 



4. Facial sutures carried well forward. 



5. Opposite portions of the circum-glabellar furrow nearly 



parallel and converging only slightly forward. 

 The writer therefore believes in the validity of Raymond's 

 new genus. 



