138 G. R MATTHEW : ILLUSTEATIONS OF 



downward in the outer half; each is traversed by a fnrrow, broad and open in the inner 

 part, but narrow and deeper beyond ; the extremities of all the pleurœ are rounded. 



When compared to other parts of the body, the pygidium is quite small. The rachis 

 has two narrow rings in fi'out, and terminates in a longer posterior lobe. The side lobes 

 are flattened, but exhibit one distinct costal rib. 



The above description applies to the narrow form in which the glabella is one-fifth 

 longer than wide. Among these narrow heads is a variety having the front margin 

 somewhat angulated, as in Solenopleura stenometopa, Aug. (See PI. I, fig. 3 b.) In the wide 

 form in which the glabella is somewhat wider than long, the width is carried well down 

 through the rachis of the thorax (see PL I, fig. 3 c); in this form, the rachis is as wide as 

 the side lobes of the thorax, and has short spines on the fifth, sixth and seventh rings. 



Sculpkire. — The shield in this species is elegantly ornamented with sharp, elevated 

 and gracefully curved ridges, and the surface of the test is minutely roughened, appar- 

 ently, with punctures which may be seen with a strong lens, but to the naked eye it 

 appears smooth. 



Length of the body, 25 mm. Width, 20 mm. Length of head to the ti]i of the occi- 

 pital spine, 10 mm. ; a very large head-shield, which is 16 mm. long and 18 mm. wide 

 between the eyelobes. Length of thorax, 14 mm ; width, 12 mm. Length of pygidium, 

 IJ mm. 



Horizon and Locality. — In the fine grey shales of Div. 1 à (" lamellatus shale ") at 

 Portland ; also at St. Martin's, in the shales of Div. 1 c, where the narrow acuminate 

 variety was found. 



This trilobite appears to have no representatiA^e in the Cambrian system in Europe. 



Conocephalites neglectiis of Hartt appears to be only a distorted example of this species, 

 that has been shortened by pressure. 



The surface of the shield in this species is excavated like Anomocare excavatum, Aug., 

 but it is clearly not that species. That has spined free cheeks, fewer joints in the thorax 

 and a much larger pygidium ; it occurs at a higher horizon in the Cambrian system. 



LlOSTEACUS OUANGONDIANUS (PI. I, figs, i a to q, and PI. II, figs. 1 a to/.) 



Conocephalites Ouangondianus, Hartt, 1868. Acad. Greol., 2ud éd., p. 648. 



C. Aurora, Hartt, 1868. Loc. cit., p. 653. 



C. quadratus, " " " " " 654. 



Plychoparia Ouangondiana, Walcott, 1884. U.S. Geol. Surv., Bull 10, p. 3t. 



P. Ouangondiana, var. Aurora, Walcott, 1884. U.S. Geol. Surv., Bull 10, p. 38. 



P. quadrata, Walcott, 1884. U.S. Geol. Surv., Bull 10, p. 39. 



This species is one of the most difficult to deal with among those of the fauna 

 described by Hartt. The head-shield presents a wide range of variation, especially in the 

 form of the anterior margin, and also in the form of the glabella ; the ocular fillet also and 

 the glabellar furrows vary much in distinctness. It is possible that there may be more 

 than one species in the several varieties herein grouped under L. Ouangondianus, but after 

 a careful comparison, the author has not been able to find characters which are constant, 

 and sufficiently distinct to form a basis of specific diagnosis. The form taken as the type 



