126 G. F. MATTHEW: ILLUSTEATIONS OF 



compared with that in front of it. In support of this fact I may name the Paradoxides 

 Acadicus, Eleminicvs, Micmac, lamellalus and regina (n. sp.) of Baud \c as compared with P. 

 Ahenacus of Band ^(l, and P. Tesdni and. others iu Europe, while the highest expression of 

 this featiire is found iu P. Davidis and P. Forrhammeri, which are the final species of true 

 Paradoxides iu Europe, but have not beeu found iu the Acadian area of Cambrian rocks. 

 The extended posterior stretch of the suture, especially where it has a strong outward 

 ilexure, is evidently connected with loug pleurœ, and these characterise the pelagi(; forms 

 of the several families of our Cambrian fauna, and it may, therefore, be asserted that the 

 variations of the suture iu this respect may be due only to the replacing of the species that 

 delighted iu the shallows of the coast by those whose home was in the open sea ; however 

 this may be, the facts remain that the earliest crustaceans of the Cambrian period, with 

 which we are acquainted, were denizens of the shallows, and that the peculiarities of the 

 suture which are found to be most prevalent among them, are those which stand out in the 

 larval forms of even the pelagic species. 



Glabella. — The change in the form of the glabella is another criterion whereby one 

 may estimate the comparative standing of the several genera which form the subject of 

 this article: in the young of all it is characterised by narrowness, and a greater extension 

 forward than we observe in the adult forms. Want of material prevents the carrying out 

 of this comparison to the ultimate iaitial forms iu all the genera, but iu several it may be 

 traced from very early stages. Notwithstanding its narrowness, the glabella in many 

 species is more prominent in the young thau in later stages. This is known to be the case 

 in Agraulos, Liostracus, Ptychoparia and Solenopleura; hence, a loug, prominent glabella 

 may be regarded as the retention of a primitive feature by those genera which possess it. 

 Agraulos, though in the parts of the head-shield hitherto discussed, appearing to hold a 

 primitive position, falls behind all the others in respect of its glabella ; and this, perhaps, 

 is a matter of some importance, as we do not know of the typical species of this genus 

 lower down than the upper part of Band Ir. It is true that there is a species in Band lb, 

 but it departs from the type in the length of its glabella. The long and prominent 

 glabella is a mark of the adult stage iu Ellipsocephalus and subgenus Strenuella, while 

 the contrary condition is most marked in Agraulos (proper), and less so in Ptychoparia 

 and Liostracus. 



Anterior Margin. — This part of the head-shield in this family of trilobites exhibits a 

 good deal of variation, which cannot well be reduced to a rule of development, partly 

 because of certain forms which are classed as A'arieties, but which, eventually, when better 

 known, may prove to be species. In the youngest known larvœ of the species that have 

 been investigated, there is no marginal fold, or raised rim with a defined furrow behind it, 

 which can be regarded as marking off the fold ; in these young individuals, the anterior 

 part of the cheeks is full, round aud prominent, and finds its best representative among 

 the adult forms in Agraulos, but it differs from that genus in that the cheeks are divided 

 from each other by a long glabella; the conditions in the young seem, therefore, rather to 

 be represented by Ellipsocephalus, iu which a long glabella nearly reaches the narrow 

 margin. There is one important feature, however, in which these two genera (Agraulos 

 and Ellipsocephalus) agree, but differ from all the others : the front margin of the shield, 

 between the genal angles, is in nearly the same plane all around, whereas in the other 

 genera, the head is arched upward at the front. A level margin is common iu the known 



