Prof. H. H. Swinnerton — Classification of Trilobites. 539 



Beecher 1 placed Remopleurid.es alongside Paradoxides. If allowance 

 be made for its Aeglinoid type of adaptation to nocturnal habits 2 the 

 similarity of this genus and its allies to the Paradoxidae and 

 Mesonacidae is evidenced by such features as the close proximity of 

 the long crescentic eyes to the glabella, the comparatively wide free 

 cbeek, the small pleural lobes, and the telson-like pygidium. 



These four families — Mesonacidae, Paradoxidae, Zacanthoidae, and 

 Remopleuridse — thus constitute a compact and natural group forming 

 a sub-order of the Opisthoparia, to which the name Mesonacida may 

 be given. It should be noted that true facial sutures have come into 

 existence within the limits of this sub-order. 



In Marella the great spines borne by the carapace and possibly the 

 marginal position of the eyes are adaptations to planktonic mode of 

 life. 3 Allowing for this, if the genus ' foreshadows ' any one type 

 more than another that type is Nevadia, for it possesses a long series 

 of segments apparently without pleural lobes and ending in a telson. 

 On the other hand, JVathorstia, as shown by the character of its 

 glabella, the width of the fixed cheek region, and the great size of the 

 pleural lobes in the trunk and tail segments, is a persistent member 

 of the stock which foreshadowed the type of Trilobite represented 

 by Conocoryphe, Ellipsocphalus, and Burlingia. In it, however, 

 caudalization is far advanced for so lowly a form. If with the 

 exception of the tail-region this Trilobite preserves the primeval 

 condition of Trilobites other than Mesonacidae, the absence of facial 

 sutures in it implies that true sutures have developed independently 

 at least three times, viz. among Mesonacida, among other Opisthoparia, 

 and among Proparia. 



Sub-order Conocoryphida. 



Conocoryphe and a few other genera which differ slightly from it 

 belong to the family Conocoryphidae. Beecher, 4 speaking of the 

 Opisthoparia, says, " from a phylogenetic standpoint the family 

 Conocoryphidse is at the base of this extensive order." Its narrow 

 marginal free cheeks, the diminution anteriorly and clearly marked 

 segmentation of its glabella, the presence of eyelines, the great 

 number of free segments, the micropygous condition, all indicate its 

 primitive character. 



Some of the genera included by Beecher in the Conocoryphidae are 

 now referred to that unwieldy, heterogeneous, and ill-defined family 

 the Olenidae. Two Lower Cambrian genera belonging to this family 

 call for special attention, viz. Ptychoparia 5 and Protolenus. 6 



Apart from the presence of eyes the general details of structure of 

 Ptychoparia (Fig. 3d) are either the same as or progressive develop- 

 ments from those of Conocoryphe (Fig. 3c). The most marked 

 difference is in the shape and size of the free cheeks. This is due to 

 the shifting of the ocular portion of the facial suture from the margin 



1 Beecher, op. cit., 1897, p. 191. 



2 Cf. Dollo, La Paleontologie Ethologigue, Bruxelles, 1910, p. 415. 



3 Ibid., p. 409. 



4 Op. cit., 1897, p. 189. 



5 Type species, Ptychoparia {Conocephalites) striata, Emmrich. 



6 G. F. Matthew, Trans. Koy. Soc. Canada, vol. xi, p. 144, 1893. 



