TRILOBITA. 



335 



borne six pairs of appendages of varying shape, two lateral compound 

 eyes, and two median ocelli. On the ventral surface of the thorax 

 there are five pairs of gills covered by flat plates, of which the most 

 anterior pair are very large, and form the so-called operculum (cf. 

 Limulus). The surface of the body was covered with scales. Some 

 of the Eurypterids reached a length of 6 ft. 



This order is sometimes placed near the Crustacea, but the general 

 opinion seems to be that which links them through Limulus to Arachnoids. 



Order 3. Trilobita. 



Trilobites, e.g. Calymene, Phacops, 

 Asaphus. 



Extinct forms chiefly found in Cambrian and Ordovician strata, but 

 extending up to the Carboniferous. The body as found is divisible into 



Fig. 145. — Trilobite (Conocephalites). — After Barrande. 

 k.s., Head shield ; pi., pleura of thoracic region ; py.. pygidium. 



three parts — the unsegmented head shield, often prolonged backwards 

 at the angles ; the flexible thorax of a varying number of segments ; the 

 unsegmented abdomen or pygidium. A median longitudinal ridge, or 

 rachis, divides the body into three longitudinal portions. 



Traces of limbs are only rarely preserved. In the head region there 

 are four pairs, apparently simple. Antennae have been recently found 

 in this region. The thorax and abdomen are furnished with biramose 

 appendages with long-jointed endopodite, short exopodite, and a gill 

 (or epipodite?) of varying shape. In the abdominal region the gills 

 were perhaps rudimentary. 



Trilobites are often found rolled up in a way that reminds one of 

 some wood-lice. So abundant are they in some rocks, that even their 

 development has been studied with some success. 



