350 SMITHSONIAN MISCELLANEOUS COLLECTIONS VOL. 53 



genal spines of the cephalon ; the pleurae of the sixth segment are 

 also extended in spines similar to those of the third segment, but 

 the spines of the pleurae of the seventh segment are bent abruptly 

 backward so far that they converge slightly toward the median 

 line ; a small, short spine occurs at the postero-lateral angle of each 

 of the pleurae of the first, second, fourth, and fifth segments; the 

 broad pleural furrow crosses the pleurae obliquely from the antero- 

 interior side to the postero-lateral side, and occupies the greater 

 part of the surface of the pleurae, except on the third and sixth 

 segments, where the furrow extends out on the base of the large 

 spinose extension of the pleura. 



The pygidium is not shown on the specimens studied by Dr. Peach, 

 but on two specimens collected since 1894 the outlines of a small 

 pygidium can be seen between the incurved spines of the seventh 

 segment ; it is without spines or angles ; about as long as wide at 

 its point of junction with the seventh segment; roughly rounded, 

 subtriangular in outline and marked by a transverse furrow about 

 midway of its length. The pygidium resembles that of the young of 

 Pcudeumias transitans, as shown by fig. 4, pi. 32, of this paper. 



Surface marked by a very minute network of very fine, irregular 

 elevated lines or ridges. 



Dimensions. — The largest specimen of the dorsal shield I have 

 seen has a length of 9 mm., exclusive of the pygidium and spines 

 (Dr. Peach mentions one 11 mm. in length). In an entire dorsal 

 shield 4.5 mm. in length [pi. 40, fig. 3] the cephalon is one-half 

 (2.25mm.) of the total length; width at the intergenal spines 

 2.1 mm. The proportions of the various parts of the cephalon are 

 fairly well shown by fig. 2, except that the specimen is a little 

 shortened by distortion. 



Observations. — The specimens illustrating this species are fairly 

 well preserved in a very fine, hard argillaceous shale, but most of 

 them have been more or less distorted by compression. A number 

 of specimens of the cephalon have been found, but entire specimens 

 are very rare. The relation of this species to the young of other 

 species of the Mesonacidae are discussed under remarks on the 

 genus Oleneiloides. 



Formation and Locality. — Lower Cambrian : argillaceous shale 

 interbedded in " Serpulite grit," a coarse quartzitic sandstone, north- 

 ern slope of Meal a' Ghubhais, 1,200-1,300 feet (366-396 m.) above 

 Loch Maree, 4 miles (6.4 km.) northwest of Kenlochewe in the west 

 of Ross-shire, Scotland. 



