232 bulletin: museum of comparative zoology. 



spines, those of the first segment being the longer. Barrande figures 

 an entire specimen of P. inflaius Corda, 5 mm. long, which differs 

 from //. carens only in possessing free cheeks, glabellar furrow No. 3, 

 and in having the terminal spines of the first thoracic segment reduced 

 to normal, while the second pair have increased in length. Into what 

 species this form finally developed there is no way of determining with- 

 out more material. It is interesting to note, however, that the devel- 

 opment agrees with the series described above in that the glabella 

 is first smooth, and the glabellar furroAvs are added during the nepionic 

 stages. They seem to be greatly retarded in this form, as only two 

 furrows (Nos. 3 and 4) have been formed in the largest specimen fig- 

 ured by Barrande. It may be noted that in this form, as in Hydro- 

 cephalus saiurnoides, the brim on the cranidium widens constantly 

 during the known stage of growth, though it never achieves any great 

 width. The M. C. Z., contains a single specimen of P. inflaius 

 (No. 651) about 5 mm. long from Velka in the Cambrian band of 

 Ginetz. All the other specimens recorded have come from the band 

 of Skrey, but that it does occur in the more southern band suggests 

 that it may possibly be the young of some form which in the adult 

 has only two pairs of glabellar furrows, possibly P. bohemicus (Boeck). 



The M. C. Z. contains a single minute specimen (No. 33) 1 mm. long 

 of a young "Hydrocephalus" which is in many respects quite unlike 

 H. saturnoides. (Plate, fig. 8). The specimen difi^ers from that 

 form in having the glabella narrow^, expanding forward. The occipital 

 ring and furrow are well marked, the glabellar furrows 2, 3, and 4 are 

 deeply impressed, extend across the glabella and divide it into ring- 

 like lobes. The anterior lobe, which is composed of lobes 1 and 2, is 

 transversely oval, and nearly twice as wide as that portion of the 

 glabella back of it. Furrows 1 are faintly indicated, and the frontal 

 lobe has a deep longitudinal furrow which does not reach lobe 3. The 

 palpebral lobes are long, and extend into spines behind the occipital 

 segment. The brim is narrow, and the truncation at the sides is so 

 slight that it is doubtful if free cheeks were present. This specimen is 

 about the size of the smallest specimen of H. saturnoides, and it does 

 not seem that it could represent a younger stage in the development 

 of that species, but it appears probable that it is the protaspis of 

 another form. From the outline of the posterior end of the ispecimen, 

 it seems probable that it is complete, and that the small posterior pro- 

 jection represents the proto-pygidium. 



The young specimen of P. spinosus figured by Barrande has already 

 been alluded to. Its chief interest lies in the fact that so small a 



