NO. 5 CAMBRIAN TRILOBITES 31 1 



Thorax with 7 (C. minor) or 8 (C. biibaris) segments; strongly 

 trilobed and moderately convex; pleural lobes flattened, with the 

 segments terminating rather abruptly in blunt points (C minor) or 

 rounded (C. stephenensis) ; pleural furrow broad, straight with nar- 

 row margins from dorsal furrow to where it abruptly narrows near 

 the termination of the segment. 



Pygidium semicircular in outline, moderately convex, strongly 

 lobed, and with a narrow, well-defined, slightly rounded border that 

 has two or more short, sharp marginal spines projecting from it; 

 axial lobe with 3 to 5 rings and a terminal section; pleural lobes 

 marked by 3 to 5 flat segments that are separated by shallow furrows, 

 and with only the faintest trace of pleural furrows. 



Hypostoma (pi. 55, fig. 6a) much like that of Bathynriscus. 



Surface of test punctate in varying degree owing to size and 

 number of punctae; it is often marked by fine, irregular, inosculating 

 ridges that may be sharp and unbroken or broader and carrying small 

 tubercles so as to give a granular appearance to the surface (C. 

 huharis, p. 314). 



Dimensions. — Most of the species are small. Corynexochus 

 bubaris, the largest, reaches 25 to 30 mm. in length, and C. minor 

 rarely more than 5 mm. 



Genotype. — Corynexochus spinulosus Angelin [Pal. Scandinavica, 



1854, p. 59]- 



Stratigraphic range. — Corynexochus senectus, C. brennus, C. 

 bubaris, C. clavatus, and C. capita occur in the upper portion of the 

 Lower Cambrian on the eastern side of the North American Conti- 

 nent. From the Middle Cambrian, C. spinulosus, C. bornholmiensis, 

 C. delagei from northwestern Europe, C. minor from eastern New- 

 foundland, and C. stephenensis from eastern British Columbia and 

 southern Idaho. 



Geographic distribution. — This has been outlined above under 

 Stratigraphic Range. 



Observations. — It was not until Gronwall published figures of 

 Corynexochus spinulosus in 1902 that the type of genus was made 

 known so that it could be compared with other forms. Angelin's 

 figures are too diagrammatic, as may be seen by comparing them with 

 Gronwall's. The cranidium of Corynexochus is somewhat similar to 

 that of Dolichometopus, but a comparison of the figures of the types 

 of the two species (compare figs. 3, 4, pi. 50, with figs, i, la, pi. 55) 

 shows marked differences. Corynexochus also differs from Bathyu- 

 riscus (pi. 46, figs. 2, 2a-b) in its glabella, fixed cheeks, thorax, etc. 



