BRITISH FOSSILS. 



Decade VII. Plate VI. Fig. 1, 2, 3. 



ACIDASPIS JAMESII. 



[Genus ACIDASPIS. Morchison. (Sub-kingdom Articulata. Class Crustacea. Order 

 Entomostraca. Tribe Trilobitse or Palaeadge.) Capable of rolling up, or even contractile 

 into a ball. Head short, broad, truncate in front ; the glabella broadest at the base, with 

 a median portion strongly separated from the three lateral lobes, which are obscurely 

 divided from the cheeks (and often connate with them) ; cheeks thickened, generally 

 ipinose at the margin, and with the angle produced into a spine ; ?yes smooth, convex, 

 (occasionally elevated on a long peduncle) connected with the front of the glabella by a 

 strong ocular ridge ; neck segment much enlarged, and generally produced into spines ; 

 body of 9 or 10 segments (fewer during the metamorphosis), with a narrow convex 

 axis, and horizontal pleurse which are produced at their ends into spines ; tail small, axis 

 abbreviated, limb multidentate, with one strong lateral rib on each side produced beyond 

 the margin, cwcty, mucro ; aamsj scutum.] 



Diagnosis. A. latus^ depressus, granulosus; capite haud convexo, 

 glabella triangularis utrinque lobis duobus ovatis d gena dilatata fere dis- 

 tinctis — tertio obscuro; oculis medianis ; jugo oculari obscuro ; angulis 

 brevispinosis ; thorace segmentis 9 unispinosis, caudd spinis 12, primariis 

 fortibus parallelis, reliquis minutis^ — terminalibus sex, externis utrinque 

 duobus. 



Synonyms. Acidaspis bispinosus (M'Coy), Salter (June 1848), Me- 

 moirs Geol. Surv., vol. ii. pt. 1. pi. 9. fig. 5 (notf. 4.) Acid. Jamesiiy id, 

 (1852), Proceed. Brit. Assoc., p. 57. 



Wo have now sufficient materials to illustrate completely an 

 Acidaspis from the Lower Silurian rocks ; they are very rare in 

 these formations in Britain, nor are they characteristic of them 

 in other countries, although they are plentiful in the Upper 

 divisions. 



The honour of first distinguishing this most remarkable genus is 

 divided between Dr. Emmrich and Sir Koderick I. Murchison ; the 

 former having fully characterized the genus* but a very short time 

 after the publication of the " Silurian System," in which the com- 

 plete head of the more common Wenlock species was figured, and a 

 new genus proposed to mark its peculiarities. 



* De Trilob. Dissert, inauguralis (1839), 53, Berlin. 



[vii. vi.] 7 F 



