Description of Genera and Species. 47 



longer and the exterior branch relatively shorter and narrower. The internal branch is 

 represented by its central keel only, which is narrow and short. Fig. 6 shows the tail fan 

 spread. In Fig. 6 the telson is wanting, and the two external branches of the viropods 

 are pressed against one another. 



Remarks. — The only form that this species could be confounded with is P. stocki; 

 hut it is easily distinguished from that species by smaller size and relatively more 

 slender proportions, and the more elongated form of telson and relatively shorter uropods. 

 The disposition of the spines and keels on the tail sufficiently marks it off from the other 

 species. 



Locality. — Eiver Esk, Glencartholm, near Langholm, Dumfriesshire. 



Horizon. — Scorpion bed, (Scottish) Calciferous Sandstone Series. 



Collector. — A. Macconochie. 



Pekimectukcs communis sp. nov. PI. VII., figs, l-.x 



S/ii'cifii- Characters. — Body elongated, 70-100 mm. (3-4 ins.); slightly applanated; 

 carapace longer than broad and without keels behind the cervical fold which crosses 

 the ba(;k about halfway between anterior and posterior margins. Tail long, massive, 

 tapering, and without keels, but with small median spines near posterior margins of last 

 two segments. External branch of uropods oidy a little longer than the hastate telson. 



General Description. — This handsome form has the lial)it of arranging for its 

 obsequies common to the members of the genus, so that, being elongated and not much 

 applanated, it is usually seen in side view and with the carapace tilted up and doubled 

 together. As shown in fig. 4, natural size, the trunk makes up about one-third of the 

 length, the other two-thirds being formed b}- the massive elongated tail. This is one of 

 the very few specimens in which the carapace is in its natural position, and. looked 

 upon sideways and slightly from above, the body is seen to be fusiform and lish-like, 

 .'ittaiiiing its greatest breadth and depth at about tlie second or third tail segment and 

 tapering more backwards than forwards. The carapace is of the usual sub([uadrate 

 form, a little longer than broad, with rounded posterior angles and emarginate posterior 

 border. Its only ornament is the thickened margin. The cervical fold divides it into 

 two nearly equal areas. No longitudinal keels have been observed behind the fold, and 

 it is doubtful whether the fold of test that is found in front of it on the crest of the 

 gastric portion represents a keel or is only an accidental wrinkle of the test. The rostrum 

 is either divided off by a suture or is articulated with the carapace. It is Inroad at the 

 base, suddenly narrowing, and continued as a blade-shaped downward-curved spine. Two 



