[MATTHEW] STUDIES ON CAMBRIAN FAUNAS 63 



Except the dwarf and peculiar genera Agnostus and Microdiscus, 

 we find that the earliest trilobites all had short ^jygidia. That the 

 pygidium was insignificant as compared with the rest of the body, and 

 perhaps more perishable, is shown by the fact that for 100 heads of 

 trilobites of the Protolenus Fauna only one pygidium was recovered. 

 Somewhat similar conditions prevailed in the Lower Paradoxides Fauna, 

 though here the pygidium begins to assume more importance. When 

 the Upper Paradoxides beds are reached, a few, and mostly small species 

 appear, in which the pygidium become of greater relative size, such as 

 the genus Dorypyge. These conditions continue until the Ordovician is 

 reached, when we find a type of trilobite becoming dominant in which 

 the pygidium equals or excels the head-shield, such are Asaphus and its 

 allies. So important do the tail-shields become that they exceed the 

 head both in size and durability. When Asaphi are collected from the 

 Utica slate where they are common, more pygidia than heads are 

 found. Although this feature of occurrence is not noticeable in the 

 Ogygia of Mount Stephen, we do observe that here for the first time in 

 the Cambrian is a genus in which the pygidium is longer than either the 

 thorax or head-shield ; it would be possible, therefore, so far as the 

 comparative size of the three parts of the body is concerned, for this 

 species in its enrolment to cover the head-shield with the pygidium. 



Not only by taking individual genera but by taking the whole 

 assemblage of genera of a fauna, the same progression in the growth of 

 the pygidium at the expense of the thorax may be noted. Thus taking 

 the genera of the Lower Paradoxides Beds which is the typical fauna of 

 the Lower Cambrian,' it appears that there is an average of 4 joints 

 to the thorax for one in the pygidium. Then taking the genera of the 

 Peltura Fauna,- which is typical for the middle of the Upper Cambrian, 

 there is an average of 2^ joints in the thorax for one in the pygidium. 

 Applying this method of comparison to the fauna of Mount Stephen, it 

 appears that its genera^ have a still more modern aspect, for they give 

 an average of only 1-7 joints in the thorax to one in the pygidium. If we 

 eliminate on one hand the Ordovician genus Ogygia, and on the other 

 Ptychoparia, a survival of the Lower Cambrian the proportion is Httle 

 changed, for it is 1*6. 



From other points of view we see a progression in these faunas, con- 

 densation of parts is supposed to be a mark of progression, and this 

 appears in the average number of somites in the thorax and pygidium. 



1 Paradoxides Anopolinus, Solenopleura, Ptychoparia, Liostracus, Conocoryphe 

 and Ctenocepalus. 



2 Peltura, Leptoplastus, Parabolina, Acerocare, Eurycare, Ctenopyge and Sphser- 

 ophthalmus. 



3 Ogygia, Bathyuriscus, Zacanthoides. Olenoides, Corynexochus, Dolichome- 

 topus, Oryctocephalus, Dorypyge and Ptychoparia. 



Sec. IV., 1899. 5 



