Prof. H. H. Swinnerton — Classification of Trilobites. 543 



lines of Asaphid development, one of which culminates in the 

 Illsenidse. Woodward ' draws attention to the close resemblance of 

 JBathyuriscus from the Middle Cambrian to the young of Ogygiopsis, 

 a resemblance which proves the Bathyuridse to be an early offshoot 

 of the ancestral Asaphid stock. Though the requisite connecting 

 links are missing, this stock must probably be looked for among 

 the Ptychoparidae, for Bathyuriscus differs from the latter only in the 

 degree of progressive development such as the broadening of the 

 glabella and axis, the shifting of the ocular portion of the facial 

 suture closer to the glabella, the more advanced caudalization, and 

 concomitant reduction in number of the free segments. The three 

 families, Illsenidae, Asaphidse, and Bathyuridaa, constitute the 

 isopygous section of the Ptychoparian type. In the more specialized 

 members the post-ocular facial suture shifts away from the genal 

 angle, thus pursuing a course divergent from that seen in the 

 Calymmenidse and the Homalonotidag. In every case this is associated 

 with a shifting of the eye, not only towards the glabella, but also 

 towards the posterior margin. This specialization proceeds quickly 

 within the Bathyuridaa and less quickly in the main branches of the 

 Asaphidse. 



All these families whose descent may be traced back to the 

 Conocoryphid type of primitive Trilobite may be placed in the sub- 

 order Conocoryphida, which may be broken into four sections, 

 viz. Conocoryphina, Olenina, Ptychoparina, and Calymmenina. 



Sub-order Tkinucleida. 



The three families, Trinucleidae, Raphiophoridse, and Harpedidse, 

 are usually grouped together. The first two closely resemble one 

 another in the short trunk and tail region, with only five or six free 

 segments and the pygidium, which is wider than long. The last is 

 linked on to the first through the Trinucleid genus Dionide, which, 

 as shown by Reed, 3 has many points in the head-shield which 

 indicate its intimate relations with Harpes, Harpides, and Erinnys. 

 The remainder of the body, however, retains the primitive micro- 

 pygous condition with numerous free segments, which decrease in 

 size posteriorly. Here again, however, Dionide bridges the gap, for, 

 whilst some species, e.g. Dionide Richardsoni, 3 have an almost 

 typically Trinucleid trunk and tail, others, e.g. Dionide Lapworthi, 3, 

 are like Harpes in outline, and though the pygidium is large show 

 clear indications of numerous segments. 



Whilst Harpes shows the type of trunk and tail possessed by the 

 ancestral Trinucleid, the characters of the ancestral head-shield 4 

 must be reconstructed from other genera; thus, Ampyx retains its 

 narrow marginal cheeks, Orometopus its eyelines and compound eyes, 

 and both show that it did not possess a broad margin or limb, various 

 species of Tretaspis and Ampyx retain indications of its segmented 



1 Geol. Mag., 1902, p. 532. 



2 Geol. Mag., 1912, p. 202. 



3 Vide P. E. C. Eeed, Monogr. Palseont. Soc, 1903, pi. iv. 



* It is conceivable that the absence of facial suture in Harpes is a case of 

 retention of protoparian conditions. 



