544 



CRUSTACEA. 



ward). The thorax is unknown, but the pygidium is large, and 

 the segments of the axis are beaded. 



Family 18. Trinucleid^e. — In this singular family (fig. 406) the 

 head-shield is enormously developed, with a wide margin or " limb," 

 which is usually perforated by rounded pores. The glabella is well 

 marked, but eyes are usually wanting, and the facial sutures may be 

 absent. The body-rings are reduced to five or six in number, with 

 grooved pleurae ; and the tail is wide and sub-triangular. The 

 family contains three principal genera, viz., Trinucleus^ Dionide, and 

 Ampyx, all of which are Ordovician or Silurian in their range ; and 

 its zoological affinities seem to be with the Harpedidce. In the 

 well-known and widely-distributed genus Trinucleus (figs. 406, 407) 

 the body is distinctly trilobed ; the " limb " of the head-shield is 



Fig. 406. — Trinucleus Pongerardi— Or dovician. The right-hand figure represents 

 a vertical section of a rolled-up specimen. 



composed of two lamellae, and is perforated by numerous larger or 

 smaller foramina ; and the " genal angles " are prolonged into con- 

 spicuous spines which are usually single, but are forked in T. Ponger- 

 ardi. The glabella is prominent and pyriform, with mere traces of 

 lateral grooves, the facial sutures being rudimentary, and the cheeks 

 being tumid, and generally furnished on each side with a small 

 tubercle seemingly representing the eyes. There are six body-rings ; 

 and the tail is triangular, with a distinct axis, and having its margin 

 entire and striated. The genus Trinucleus is wholly confined to the 

 Ordovician rocks, and is specially characteristic of the Bala beds. 

 The genus Dionide has a sub-quadrangular glabella ; and the " limb " 

 of the head-shield, though perforated, is rudimentary, and by absence 

 of the facial sutures becomes continuous with the cheeks. The eyes 

 are wanting, and there are six body-rings. In the curious Ordovi- 

 cian and Silurian genus Ampyx (fig. 408) the head is sub-triangular, 



