1916] The Ottawa Naturalist. 57 



of note. Schmidtiellus mickwitzi (Schmidt) (a), with its thirteen 

 segments, absence of any enlargement of the third, and the 

 presence of a spine on the eighth segment, is clearly distinct 

 from Mesonacis, but its relationships are obscure. Of the 

 genera in which there is no tendency toward resorption (Wati- 

 neria, Holmia, and Callavia), Callavia (b) alone shows a ten- 

 dency toward an enlargement of the third segment. The 

 genera showing resorption (Mesonacis, PaeJeumias, and Olenel- 

 lus) all have an enlarged third segment. This is also true for 

 Elliptocephala (c), though the differentiation between the third 

 and other ribs disappears in this species in the adult. In 

 Olenelloides (d), a bizarre survivor of Mesonacidae, the third 

 segment is enlarged. 



The enlargement of the third segment appears to be im- 

 portant from a morphological standpoint, and it is preserved 

 among Middle Cambrian trilobites bearing relationships to the 

 Lower Cambrian Mesonacidae in the youthful forms of Zacan- 

 thoides (e), and the adult forms of Albertella helena (/). Its 

 importance in the latter species is, however, largely negatived 

 by the fact that in the very closely related Albertella bosworthi 

 (g) it is the fourth segment which is enlarged. In both species 

 the total number of segments is the same, seven, but the number 

 of segments uniting to form the tail is larger in bosworthi than 

 in helena. The second segment is enlarged in the young of the 

 following species of Paradoxi ies : hohemicus Boeck, injlatus Corda, 

 lyelli Barrande, nigulosus Corda, and spinosus Boeck. In Hydro- 

 cephalus car ens, H. saturnoides and Paradoxid^s pitsillits Barrande 

 the anterior two segments are enlarged. In Shumanlia pusilla 

 (Sars) the fourth ssgment is large, irrespective of the number 

 of segments between the fourth and the tail. In several species 

 of Cybele it is the sixth pair, and Cyphaspis barrandei and C. 

 burmeisieri are each characterized by the presence of a very long 

 median spine on the sixth segment. In one species of Illaenus 

 {hisingeri Barrancle) it is the first. In Bathynotus it is the 

 eleventh and last, iviedian thoracic spines have been described 

 for the following Cambrian species: Saratogia hera Walcott (h), 

 Norwoodia tenera Walcott (/), and A T . gracilis Walcott (/)'. Za- 

 canthoides, which has been mentioned as one of the two Middle 



(K) Moberg in Moberg and Segerberg, 1906, Kongl. Fvsiog. Sallskapets Handl., N.F., Bd . 



17, 1906, p. 35. 

 (6) Walcott, Smithsonian Misc. Coll., vol. 5 7, No. 11, 1913, pi. S3, figs. 1 and 3. 

 (c) Walcott, Smithsonian Misc. Coll., vol. 53, No. 6, 1910, p. 269. 

 (il) Peach, Quart. Jour. Geol. Soc. London, vol. 50, pp. 669-670, pi. 32, figs. 1-6. 

 (e) Walcott, Smithsonian Misc. Coll. vol. 53, No. 2, 1908, pi. 3, figs. 5 and 10. 

 (/) Idem, pi. 2, fig. 8. 

 ('/.. Idem, pi. 1, fig. 5. 



(h) Smithsonian Misc. Coll. vol. 64, 1916, pi. 35, fig. 3b. 

 (i) Idem, pi. 2 8, fig. 2d. 

 (j) Idem, pi. 27, fig. 2f. 



