﻿Principal Daivson — On New Pakeozoic Crustacea. 



57 



It is associated with shells of Naiadites carhonariiis and N. elongatus 

 [Anthrucomya of Salter) and with cara- 

 paces of Cythere and Bairdia. 



The specimen is a flattened carapace, 

 without any of the other parts. Its 

 length, without the rostrum, is 0-9 inch. 

 The rostrum projects 0-25 inch. The 

 extreme breadth is 0-85 inch. The 

 surface is smooth at the sides, but pa- 

 pillose in the median portion. As com- 

 pared with the published figures and 

 descriptions of P. duhius, its distinctive 

 characters appear to be : — 1. The shorter 

 rostrum and larger spines at the anterior 

 angles. 2. The reduction of the denta- 

 tions on the anterior part of the sides to 

 five in number. 3. The presence of two 

 strong spines in front of the cervical 

 groove at each side of the base of the 

 rostrum, and parallel to it, and rather 

 less than half-way between it and the 

 lateral margins. Whether the dorsal 

 ridge extended to the posterior margin 

 cannot be seen, as this part of the crust 

 is imperfect. In other respects it re- 

 sembles the British species, and may be 

 merely one of its varietal forms. 



Fig. 2} HomaJonotus Pawsoni, Hall. 



This species was described by Prof. Hall in the Canadian 

 Naturalist in 1860, from specimens of the pygidium and portions of 

 the body segments, collected at Arisaig, in Nova Scotia. The 

 cephalic shield was afterwai'ds found at the same place, and was 

 described and figured by Dr. Dawson, in " Acadian Geology," second 

 edition, 1868. The present specimen, showing the entire length of 

 the body, with the exception of a small part of the posterior 

 extremity, was found by Mr. D. Eraser, in the Upper Silurian slates 

 of the East Eiver of Pictou, Nova Scotia. 



The specimen is a cast of the inner surface of the crust, so that the 

 deep transverse furrows are not the margins of the segments, but 

 the impressions of their strong internal ridges. The ends of the 

 pleurae are bent down abruptly at right angles to the back, giving it 

 a very narrow appearance when viewed from above ; but this may 

 be partly a result of distortion, in connexion with the slaty cleavage 

 which has affected the rock. Still, from the great width of the seg- 

 ments, the thorax is of great length, in comparison with the 

 cephalic shield and pygidium, and this seems to be one of the 

 distinctive characters of the species. The caudal shield, as described 

 by Prof. Hall, has, in all, nine annulations ; of which in this specimen 

 ^ From a Photograph by Henderson, reproduced by ' Dallastint.' 



Fig. 2. 



