324 Shield-hearing Crustacea. 



(Figs. 1 and 2), and by four fossil forms which carry us back in 

 time, first, to the period of the deposition of the Lithographic 

 Limestone, and the era of the Archceopteryx* and Pterodac- 

 tyles,f with long tails (Fig. 3) ; and then, still further back, 

 to the great Carboniferous epoch, when the Coal-measures 

 were accumulated (Figs. 4, 5, 6). 



Lastly, the Eurypterids, which are here represented by a 

 newly described genus J (Hemiaspis, Fig. 7) from the Lower 

 Ludlow Rock of Leintwardine, Shropshire, and have been illus- 

 trated on a former occasion in this work by the genus Slimonia§ 

 from Lanarkshire. 



In a chart of Fossil Crustacea, || recently engraved by Mr. 

 J. W. Lowry (and to which we are indebted for Figs. 3 — 9 of 

 our plate), we are for the first time introduced to a general 

 view of this ancient class of Articulate animals. 



By a reference to this chart it will be seen that the oldest 

 order in time is that of Trilobites, which are characterized by 

 the trilobed form of the trunk- segments (as well seen in Harpes, 

 Fig. 8) , the possession of sessile compound eyes — in most — and 

 the entire absence of limbs of any kind. With nearly all 

 species is found an articulated labrum, or lip-plate, attached by 

 its anterior margin to the under-side of the front border of 

 the head. 



The rest of the animal must have been clothed in thiu mem- 

 brane only, as up to the present time no trace of limbs or other 

 organs has rewarded the diligent search of Barrande, Salter, 

 Angelin, and a host of others who have essayed to find them. 



The next oldest group will be found to include the bivalvcd 

 and shield-bearing forms, the Ostracoda and Phyllopoda ; the 

 former represented by the little Gypris, so abundant in all our 

 freshwater ponds, and commonly to be seen in every aquarium ; 

 the latter by the recent Apus (Fig. 10) and the fossil 

 Dithyrocaris (Fig. 9). 



Beneath the shield- shaped covering can be seen an articu- 

 lated labrum attached, as in the Trilobites, to the under-side of 

 the frontal portion of the carapace. Against the sides of this 

 labrum the jaws, armed with serrated teeth, are attached, by 

 means of which the food is triturated. Then follows the lower 

 lip, as it is called, and next a pair of branched and finely arti- 

 culated swimming feet, followed by a series of about thirty 



* See the Intellectual Observer for December, 1862,vol. ii. p. 3] 3, and plate. 



f See the Intellectual Observer for January, 1863, vol ii. p. 443. 



j See Quart. Journ. Geol. Soc, Nov. 1S65, vol. xxi. p. 4U0, plate 14, fig. 7. 



§ See the Intellectual Observer for Nov. 1863, vol. iv. p. 229, plate and 

 woodcuts. 



|| A new chart of Fossil Crustacea (accompanied by a descriptive Catalogue), 

 arranged and drawn by J. W. Salter, F.G.S., and Henry Woodward, P.Q-.S., F./..S. 

 Engraved by J. W. Lowry, published by J. Tennant, London, Sept, 1865. 



