228 bulletin: museum of comparative zoology. 



cephalon occupying three fourths of the length and the thorax and 

 pygidium one fourth. The thorax contains three segments. The 

 cephalon has three glabellar furrows which cross the entire glabella. 

 Three more stages of development are illustrated by Barrande, in the 

 last of which the test is about 2 mm. long, the cephalon being a little 

 over 1. mm. In the third, fourth, and fifth stages the first two seg- 

 ments of the thorax bear backward-directed spines and the intergenal 

 spines are still present. The most conspicuous feature of the cephalon 

 in stage five is the presence of a narrow, smooth, flat brim on the front 

 of the cranidium. This is first seen in stage four and becomes wider 

 in stage five. The three glabellar furrows and the median longitudinal 

 furrow are still present at stage five, but the median furrow is not so 

 conspicuous as in the smaller specimens. 



The ontogeny of Olenellus indicates that the palpebral lobes are 

 formed by the recurved pleura of the second glabellar lobes. It is 

 very important to note that in these specimens known as Hydro- 

 cephalus saturnoidcs the anterior ends of the palpebral lobes join the 

 glabella in front of the anterior glabellar furrows, thus indicating that 

 the furrows present are 2, 3, and 4. Beyond stage five Barrande did 

 not trace any line, but the writer believes that Paradoxides orphanus 

 and P. jynsiUus represent the next stages of this same species. In the 

 M. C. Z. there is a cranidium 1.5 mm. long, identified as P. pusillus, 

 but answering better to the description of P. orphcmus, which in some 

 measure fills the gap between the largest of Barrande's figured speci- 

 mens of H. saturnoides and the smallest of his P. orphanus and P. 

 pusillus. In this specimen the anterior brim is narrow, occupying 

 about the same proportion of the whole length as is shown in Bar- 

 rande's figure of P. orphanus. Glabellar furrows 2, 3, and 4 all cross 

 the whole glabella, as in //. saturfioides instead of 3 and 4 only, as in 

 P. pusillus, but the connection between the two sides on furrow 2 is 

 quite shallow. This specimen, moreover, adds another pair of fur- 

 rows at the sides just in front of the palpebral lobes, as in P. pusillus. 

 From this specimen to a typical pusillus with a wide brim, the collec- 

 tion contains all stages, so there is no doubt of the connection in that 

 direction. Barrande has figured {Loc. cit., 1872, 1, suppl. pi. 9, 

 fig. 22, 23), an entire specimen of P. pusillus 2.5 mm. long, the cepha- 

 lon of which makes up 55% of the length. Seven free segments are 

 present, and the pygidium contains three or four more. The cephalon 

 has a wide brim, 23% of the whole length, and there are no intergenal 

 spines present, though the first two segments of the thorax have long 

 spines, the spines of the second being longer than the first. 



