124 G. F. MATTHEW : ILLUSTRATIONS OF 



treatment of zoological questions connected with the earliest deposits of the Cambrian age 

 — owing to the alteration to which most of the sediments of this time have been subjected, 

 and to the poverty of the fauna in most districts — are obstacles to the prosecution of these 

 researches ; and the great divergence of existing forms of animal life from those which 

 peoxjled the primeval seas is such as to induce the student to search among the rarer and 

 the extinct types for the relatives of these past existences, or to seek in the embryonic 

 characters of the young for the links by which a natural classification of these oldest 

 primordial forms may be obtained. 



While the former method has been found prolific of good results in making clear the 

 relationship of types of animal life, now rare or extinct, with tribes which once abun- 

 dantly peopled the earth ; the latter has been no less useful in shewing the relations of 

 the Cambrian genera to each other, and to kindred organisms of later date. It is chiefly 

 through the latter method, and by the consideration of the adult form of these small trilo- 

 bites, that I have endeavoured, in the following division of my paper, to classify them as 

 well as the imperfect material at hand will allow. 



When the late Prof. C. F. Hartt described the species of Band c in Div. 1 of the 

 St. John group, he included under Barrande's genus Conocephalites, a number of species 

 which palaeontologists, in later years, have, for obvious reasons, found it necessary to dis- 

 tribute to seA'eral genera.' Of the species classified by Prof. Hartt, under the above genus 

 of Barrande, the writer has already described the larger forms in which the eyes were 

 absent ; these belong to the subfamily Conocoryplmue. There remain, however, for des- 

 cription, the smaller species furnished with distinct eye-lobes, and these I propose to 

 describe in the following pages. 



The difiiculties surrounding the treatment of this group of species have been materially 

 reduced by the publication of Mr. C. D. Walcott's Bulletin No. 10. of the series of Bulletins 

 issued by the United States Geological Survey. In this bulletin are excellent figures of 

 the trilobites from the St. John group, which Prof. Hartt used in describing the species 

 originally published iu " Acadian G-eology." Mr. Walcott regards a number of Mr. Hartt's 

 species as varieties of his other species, and in this determination, after examining other 

 and independent series of forms, I fully coincide ; though I incline to the opinion that 

 other genera besides Ptychoparia are included in these forms. 



This view has been arrived at after a study of more complete examples of several of 

 the species than are to be found in the Hartt collection, studied by Mr. Walcott, and now 

 preserved iu the Museum of Cornell University at Ithica, N.Y. These more complete 

 individuals have thrown new light on the classification of these small species, and have 

 enabled the writer to compare the Acadian fauna to better advantage with that of the cor- 

 responding horizons in ScandinaAÙa and Wales. It is true that we have still to regret the 

 imperfection of our knowledge of a number of these species, which only the recovery of 

 the missing parts can remove ; but of other species we know enough not only to improve 

 upon the classification of these forms which has hitherto been used, but also to throw 

 light upon the relationship and geological position of the species of which our knowledge 

 is more imperfect. 



' iVmong other reasons for dividing this genus, it may be mentioned that Mr. C. D. Walcott enumerates ninety 

 species of American Cambrian trilobites which would fall under it. TJ. S. Geol. Surv. Bull. 30, p. 61. 



