Shield-bearing Crustacea. 325 



pairs of leaf-like appendages (gill-feet) which serve the office 

 of branch! as, and by their constant vibratory motion enable the 

 animal to move freely through the water. When the shield is 

 removed, the body is seen to be composed of a head bearing 

 the eyes and thirty-three segments, terminated by a telson (or 

 tail-plate), bearing two long many -jointed appendages. The 

 first thirteen segments are quite soft and worm-like on their 

 upper surface, being protected beneath the broad head-shield, 

 the rest are ornamented with six or eight little spines along 

 the posterior margin of each joint. AU but the last six bear 

 gills upon their under surface. The eleventh pair of feet are 

 modified so as to form egg-pockets (ovaries), for the preserva- 

 tion of the eggs until the young are hatched. 



We find traces of the appendages in Ceratiocaris, and in 

 both Ceratiocaris and Dithyrocaris the jaws * are preserved- in 

 a fossil state. 



There are several points in the structure of Trilobites 

 which indicate a close relationship to the Phyllopods. 



We find in both the articulated labrum, and although in the 

 former (Trilobites) we do riot find foot-jaws, it does not certainly 

 follow that they had none, although the negative evidence 

 has hitherto seemed to favour that view. . 



In both a greater number of segments is attained than is 

 usual in the Crustacean type. Thus in Conocephalus there are 

 sixteen (reckoning one each for the head-shield andpygidium); 

 in Paradoxides twenty to twenty -two ; in Arethusina twenty- 

 two ; and in Harpes (Fig. 8) twenty-eight ; and if we look upon 

 the head-shield and tail-plate as composed of several body- 

 rings joined together, which seems certain, we have forms 

 presented to us in which the multiplication of segments like 

 each other is one of its leading features, illustrating that kind 

 of growth which Professor Owen has most aptly called vegeta- 

 tive repetition of parts. 



The next oldest group in time is found to be the Eurypterida, 

 represented in our plate bv Hemiaspis (Fig. 7), and ISlimonia 

 (see Plate in Intellectual Observer, vol. iv. p. 229). 



It is unnecessary here to enter upon a detailed description 

 of SUmonia, that genus and its allies having already appeared 

 with illustrations in the article before mentioned. But 

 the genus Hemiaspis offers some new points of structure and 

 affinity which are deserving of notice. f 



The great interest attached to this new genus is, that it 

 appears to offer just the link we needed to connect the Xiplw- 

 sura (King-crabs) with the Eurypterida. 



* See my article on Crustacean teeth in the Geological Magazine for Sept., No. 

 15, vol ii. p. 401, plato xi. 



t See article in Quart. Journ. Geol. Soc, Nov. 1865, vol. xxi. p. 400. 



