358 Canadian Record of Science. 



doxides. In other respects this genus (Ellipsocephalus) 

 appears to be the most primitive type of the group of 

 species to which this paper relates ; its pleurse are short 

 and bat little bent or pointed, its glabella is long and 

 prominent , the facial suture is short and distant from the 

 glabella, and the occipital furrow is faint. 



The changes during growth in Liostracus and Ptycho- 

 paria are suggestive, and may be briefly presented as fol- 

 lows : — Omitting the first stage, indicated in all these ti'ilo- 

 bites, but which cannot be said to be special to any one, 

 namely that in which the test of the creature is represented 

 by an oval body with an umbilicus-like depression, one may 

 first speak of a phase of the growing embryo in which the 

 rachis is distinctly raised throughout its whole length and 

 the cephalic shield, and the pygidium, indicated by a strong 

 groove across the axis, about two-thirds or three-quarters 

 of its whole length, from the front of the embryo. At this 

 stage, the occipital ring is faintly outlined by a shallow 

 groove across the rachis, and a similar, but fainter groove, 

 indicates the first ring of the axis of the pygidium. The 

 cheeks of the cephalic shield and the side lobes of the pygi- 

 dium still form a continuous, rounded surface, except that in 

 some examples these two parts of the body are indicated on 

 the side lobes of the test by a faint transverse line. At this 

 stage we are unable to distinguish even the family of the 

 trilobite by the form of its test, for with its single ring in 

 the pygidial part, and the enlarged front of the anterior 

 end of the axis in the cephalic portion of the embryo, the 

 later stages may as well exhibit to us a Conocoryphean as 

 a Ptychoparian trilobite. 



In the next stage of growth in which the cephalic shield 

 and the pygidium are entirely separated, features appear in 

 the former which enable one to distinguish the trilobites of 

 this family from the Conocoryphidse, these are the less per- 

 fectly semi-circular form of the cephalic shield, the greater 

 prominence and comparatively larger size of the glabella, 

 and the presence of eyelobes. At this stage in Ptychoparia 

 and Liostracus, the dorsal suture is much nearer the margin 

 than in the adult, a point in which these young tests 



