THE FAUNA OF THE ST. JOHN GEOUP. 125 



In studying these small species it will be found that they arrange themselves under 

 the following genera : — Ellijjsocephalus, AgranJos, Ltosfracns, Ptychoparia, Solenopleura. 



The history of the growth of the trilobites of the Acadian horizon in the St. John 

 group has been pursued suHiciently far to shew that, omitting the genera Agnostus and 

 Microdiscus, the species originated in three family or tribal types ; one, that of the Para- 

 doxides; a second, including Conocoryphe and Cteuocephalus ; and a third, that of the 

 group to which this part of the paper relates. In previous articles on this fauna, the 

 writer has described the chief characters of the development of the head-shield in the first 

 two groups, and now proposes to deal with the third. The differences between Para- 

 doxides and the other two groups are plainly exhibited in the youngest tests, but it is not 

 so easy to separate the heads of the young of the other two families, Conocoryphidfe and 

 Ptychoparidœ, when they are first marked by the transverse ridge which indicate the 

 origination of the head-shield and pygidium as separate parts of the organism. The 

 distinction, when first observable, is to be recognised in the following way : in the 

 CouocoryphinsB the head-shield, at first oval, begins to assume a semicircular form, and 

 we observe that the glabella is not so large in proportion to the rest of the head-shield, 

 nor so prominent, as in the young of the third group, especially in Liostracus and Ptycho- 

 paria. Shortly afterwards, the distinction in these respects is emphasised by the appear- 

 ance of the eye-lobes near the lateral margin of the head-shield of the trilobites of the 

 third group, and it is seen that we have to deal with species which are provided with 

 eyes. The marginal eyes continue to be a permanent feature in EUipsocephalus and 

 Agraulos, but are gradually indrawn toward the glabella in the other genera as they 

 continue to develop. In the genera last-named (Ptychoparia and Solenopleura) we may 

 conclude that we have the highest type of development in this part, as the eyelobe is 

 with them shortest and most prominent, and, at the same time, most strongly indrawn 

 towards the glabella. The two genera, first named, on the contrary, present in this 

 part of the shield, traces of a primitiA^e habit, in that the eyclobes remain nearly 

 marginal in the adult ; of the two, EUipsocephalus may be regarded as most primitive, 

 seeing that it is not only nearly marginal, but also of considerable length. And, further- 

 more, the eyelobe in EUipsocephalus, extending so far back as it does in the adult, tells 

 also of the comparatively independent standing of this genus, for we know of no early 

 Cambrian genus with long eyelobes iu which the posterior end of that lobe is nearer the 

 hind margin iu the adult than it is in the earlier stages. The young of three other genera 

 (Liostracus, Ptychoparia and Solenopleura), however, have this feature in common, that 

 they exhibit in their earlier stages an eyelobe which, while at first near the anterior margin, 

 gradually recedes from that part of the shield, but at no stage of growth exhibits the 

 comparatively long eyelobe of Ellipsocephalns. 



Dorsal Suture. — In this feature, which is connected with the appearance of the eyelobe 

 in trilobites, a change of form occurs parallel to that which their eyes exhibit. In the 

 earliest larA^œ of Ptychoparia and Solenopleura wherein it has been detected, it has the 

 same directness and simplicity which we observe in Agraulos, and, as iu this genus and 

 EUipsocephalus, it is short : the latter, however, possesses a characteristic which iu the 

 genus Paradoxides may be seen to be a primitive feature, and is presumably so in EUipso- 

 cephalus : this is the short and direct posterior extension of the sutui'e. In the earliest 

 Paradoxides it will be seen that the extension of the suture behind the eyelobe is short 



