644 CLASS x. 



These animals, on a first impression, seem to have some conformity with 

 the genus Limulus, although they are much smaller. They differ however 

 from these greatly, as well in their oral organs, which are formed on the 

 type of insects (compare SAVIGNY Mem. sur les Ani. s. v. I. pp. 63, 64, 

 PI. VII.) as in internal structure. They undergo, as already stated in 

 p. 613, a metamorphosis resembling that of Branchipus. They are found 

 in fresh water, canals, fish-ponds, and pools formed by abundant rain. 



Compare G. C. SCH^FFEE Der Krebsartige Tciefenfuss, mit 7 ilium. 

 Kupfertafeln. Regensburg 1756. 4to, and E. G. ZADDACH, De Apodis can- 

 criformis Anatome el Jfistoria evolutionis, acced. tabulae 4 lithogr. Bonnae, 

 1841, 4 to. 



Sp. Apus cancriformis LATR., SCH^EFFEB Monogr. cit., Elem. entom. Tab. 29, 

 figs. I. II. ; F. H. LOSCHGE Naturforscher, Tom. xix. 1783, Tab. in.; 

 GUERIN Iconogr., Crust. PI. 34, fig. i. 



Another smaller species has an elongate oval plate between the two 

 filaments of the tail, and forms the genus Lepidurus LEACH; it is the 

 Apue productus LATR., Monoculus apus L., DESMAR. Crust. PI. 52, fig. i, 

 GUERIN, IT. fig. 3. 



Appendix to the PHYLLOPODA. 

 Trilobites or Palceades. 



Fossil crustaceans. Shield cephalic, rounded anteriorly, mostly 

 supplied with two lateral eyes oval or reniform, often extending j 

 posteriorly on each side into a horned point. Segments of thorax j 

 and abdomen transverse, different in number, mostly divided by 

 two longitudinal furrows into three convex lobes. The posterior 

 part of abdomen often undivided (pygidium). Body of some con- 

 tractile into a ball. 



Very different opinions were entertained formerly respecting the 

 Trilobites. Because the transverse segments in these fossil indi- 

 viduals offered some resemblance to Chiton, LATREILLE was really oi 

 opinion that they ought to be placed in the neighbourhood of this . 

 genus. Mem. du Museum, VIL pp. 22 32, Regne Ani. 2e edit. iv. 

 pp. 202, 203, whilst he had previously considered them to be a j 

 transitional form between the Crustacea and the myriapoda, between 

 Limulus and Glomeris (R. Ani. lere e"dit. in. pp. 150, 151). That 

 these remains belong to the type of the articulates was already 

 apparent to LINNAEUS, who collected the few petrifactions of this 

 division known to him under the name of Entomolithus paradoxes. 

 WAHLENBERG in 1818 (Act. Societ. Reg. Scient. Ups. Yol. vin. p. 1), 

 in a memoir in which new species were also described, illustrated 



