236 bulletin: museum of comparative zoology. 



Eight of the hirgest cranidia in the collection, varying from 60 to 

 120 mm. long, show a longitudinal cracking along the median line 

 which strongly recalls the median longitudinal furrow of Hydro- 

 cephalus. In this case it is not exactly a furrow, but the crushing 

 along this line of so many specimens indicates a line of weakness here. 

 The backward or forward turning of the third and fourth furrows of 

 the glabella at the median line in so many species is also probably to 

 be connected with this furrow. 



Comparison with other Species. 



As mentioned above, there are only a few rimless species of Para- 

 doxides, and it is with such forms alone that P. harlani can be compared. 

 It has been compared most commonly with P. spinosus (Boeck) ; but 

 from that species it differs, not only in the possession of a rimless brim 

 and the absence of the terminal spines on the second thoracic segment, 

 but also in the pygidium, which in P. harlani is larger and longer and 

 has a much longer axial lobe than the Bohemian species. 



The species which lack the rim, besides P. harlani, are P. bennetti 

 Salter, P. groomi Lapworth, and P. regina Matthew. P. bennetti is 

 very similar to P. harlani in the shape of the glabella, the possession of 

 four pairs of glabellar furrows, and medium sized eyes. The genal 

 spines appear to be shorter, and according to the single specimen in 

 the M. C. Z., the brim is not so wide. In this specimen, the second 

 segment does not seem to be enlarged as indicated by Salter and 

 mentioned by Ford, but is actually smaller than the first. 



Paradoxides groomi is known only from fragments which indicate 

 a species similar to P. harlani, but with narrower thorax and, accord- 

 ing to Cobbold's description, shorter fixed cheeks. 



The principal differences between P. regina Matthew and P. harlani 

 seems to lie in the pygidium, which is more quadrangular in outline 

 and has a shorter axial lobe in the former species than in the latter. 

 Outside the pygidium it is, as has been pointed out by Grabau, ex- 

 ceedingly difficult to point out differences between the two species. 

 The majority of specimens of P. harlani have a narrower cephalon 

 and glabella than the Acadian form, but as Grabau has already shown 

 in his table of measurements, we have specimens of a wide form which 

 correspond very closely to the dimensions of Dr. Matthew's specimen. 

 Incidentally I might mention that the Geological section of the M. C. Z. 

 has recently acquired a specimen of P. harlani with a cranidium 138 



