MUSEUM OF COMPARATIVE ZOOLOGY. 37 



the middle. The glabella is marked by three pairs of shallow furrows, with 

 perhaps a fourth scarcely discernible pair. The second and third pairs are not 

 seen to meet across the median line; but owing to the position of two very low 

 and rather indistinct tubercles, one anterior and one posterior to the first or 

 posterior pair of furrows along the median line, these furrows seem to meet in a 

 curve bending slightly backwards along the middle of the glabella. The palpe- 

 bral lobes are large and prominent, beginning a little anterior to the third pair 

 of furrows, and curving around to within a very short distance of the exten- 

 sion of the occipital furrow across the cheeks. The curve along the anterior 

 border of the head is regular. A shallow groove runs within a short distance 

 of the anterior border, gradually becoming deeper and broader and receding 

 more from the border on the sides of the head, so that the rim here becomes 

 broader. A faintly discernible shallow pit near the anterior extremity of the 

 glabella may in this case be only accidental. 



Paradoxides tenellus, Billings, is in size like this species, but otherwise 

 very distinct. It is interesting to find a Paradoxides in the Olenellus Cam- 

 brian, since its occurrence there diminishes the importance of the Paradoxides 

 Cambrian as a Paradoxides division. 



Locality and position. — Station No. 2, North Attleborough, Mass., Cam- 

 brian, one specimen. 



22. Ptychoparia mucronatus, sp. n. 



Plate II. Fig. 31. 



Glabella in small specimens very convex, the degree of convexity decreasing 

 with the increase of size; general form oval, the anterior part becoming some- 

 what narrowed. The occipital furrow is always well defined. There are three 

 pairs of glabellar furrows. These vary greatly in distinctness, being as a rule 

 less prominent in young individuals and more marked in large specimens. 

 The anterior pair is also usually much less distinct than the other two. The 

 occipital ring is well rounded, except where interrupted by the nuchal spine, 

 which is generally present, but usually small, being a mere terminal tubercle 

 directed backwards and often more prominent in young specimens than in 

 older individuals. From this are all variations to that of a moderate-sized 

 spine. In one specimen, a fragment, the head of which may possibly have 

 been 14mm. long, the spine extended for a distance of 3.2 mm. beyond the 

 general outline of the occipital ring, the base being broad, narrowing suddenly 

 to a short slender spine. Many large specimens are found in which this nuchal 

 spine is not seen. Examination, however, always indicates that in these cases 

 the posterior extremity of the occipital ring has been injured, so that the ab- 

 sence of a nuchal spine cannot be definitely asserted. They agree perfectly in 

 every other respect with the spined forms. Considering that we have positive 

 evidence of large forms with spines, the absence of the same in specimens all 

 apparently more or less injured seems to mean little. One specimen 15 mm. 



