Prof. H. H. Swinnerton — Classification of Trilobites. 541 



Other families group themselves round the Olenidae (s. str.) and 

 the Ptychoparidae respectively. In some cases the connexion is close 

 and is evidenced by known transitional forms. In other cases the 

 relationship is distant, and only the inability to find a more suitable 

 place justifies putting them here. 



The Proetidae are a small compact family within which all stages in 

 the increase of free and reduction of fixed cheeks and in caudalization 

 may be seen. Beecher ' derives them from Arethusina, which Reed 2 

 classes with the Proetidae and Raymond 3 as one of the Olenidse. 

 Those features in which this genus differs from the Olenidse (s. str.) 

 it has in common with Cyphaspis. It seems probable, therefore, that 

 the Proetidae have arisen from an Olenid stock. 



In the family Okyctocephalid^, Raymond 4 includes Ory otocephalus, 

 Zacanthoides, Oletioides, and Neolenus,. As already seen Zacanthoides 

 belongs to the Mesonacida. Reed 5 places Dorypyge with Orycto- 

 cephalus and Olenoides. A comparison of these three genera shows 

 that they all possess the protolenus type of cheek region. Reed 2 

 further considers them related to Parabolina and Parabolinella, 

 But the latter are essentially Upper Cambrian forms in which 

 caudalization has hardly begun. The former ai'e Lower and Middle 

 Cambrian types in which caudalization has already advanced to the 

 isopygous stage. Had the order of appearance in time been reversed 

 close relationship could not have been denied ; as it stands, the 

 Oryctocephalidae must be regarded as a separate branch of the same 

 stock, a branch in which caudalization began early and rapidly 

 reached its acme. Were it not for the spines they might be 

 described as occupying the same position in the economy of the 

 Lower and Middle Cambrian as did the Asaphidae in the Ordovician. 



The Ptychoparidae, though not so large a family as the Olenidae, 

 seem to stand near the point of origin of many more families. 



The close affinity of the Solenopleuridae is too evident to call for 

 discussion. 



The Dicellocephalidae include Crepicephalits and Dicellocpphalus. 

 Of the first-named genus Walcott 6 remarks, "the essential elements 

 of the head are generically identical with those of Ptychoparia 

 striata, but the pleurse and thoracic segments and pygidium vary in 

 a marked degree." Certainly the free cheeks are only a slight 

 advance on the ptychoparian owing to the shifting of the ocular 

 suture closer to the glabella and to the posterior margin. Dicello- 

 cephalus differs from Crepicephalus chiefly in being isopygous. The 

 whole family bears the marks suggestive of the adaptation of the 

 ptychoparian type to fossorial habits. 



As shown by Pompeckj 7 the Calymmenidse and the Homalonotidae 

 are derived from the Ptychoparidae through Bavarilla and Neseuretus. 



1 Amer. Joum. Sci., vol. iii, p. 195, 1897. 



2 Op. cit., 1904, p. 74. 



3 Zittel's Textbook, 1913, p. 715. 

 * Op. cit., 1913, p. 716. 



5 Palceontologia Indica, 1910, p. 10. 



6 Cambrian Fossils of Yellowstone Park, Monogr. 32, U.S.G.S., 1899, p. 460. 



7 Op. cit., 1898. 



