THORACOSTRACA OR PODOPHTHALMATA. 



559 



As in the case of the Amphipods, the geological history of the 

 Isopods is very imperfectly known. The Palaeozoic Arthropods 

 which have been referred here, such as the problematical Arthro- 

 pleura of the Carboniferous rocks, and the form described by Dr 

 Henry Woodward from the Old Red Sandstone under the name of 

 P?-ceci7'cturus, are of doubtful affinities. The earliest unquestionable 

 Isopods are found in the Jurassic rocks 

 (the Lithographic Slates of Germany), 

 in which the order is represented by 

 the extinct genera Urda and sEgites. In 

 the Upper Jurassic (Purbeck beds) of 

 Britain is found the Archceoniscus Brodiei 

 (fig. 420), often in considerable num- 

 bers, the genus being apparently refer- 

 able to the recent family of the yEgidce. 

 To the same family belongs the extinct 

 genus Palcega, species of which are found 

 in the Cretaceous and Tertiary rocks. 

 A tolerably abundant Tertiary genus is 



Eosphceroma, which belongs to the recent family of the Sphceromidce. 

 Lastly, the terrestrial family of the Wood-lice (Oniscida) is repre- 

 sented in late Tertiary deposits by such existing genera as Armadillo^ 

 Porcellio, and Oniscus itself. 



Fig. 420. — Archceo7iiscns Brodiei, 

 a fossil Isopod from the Purbeck 

 Beds. 



Division B. Thoracostraca or Podophthalmata. 



The Crustaceans included in this division possess compound eyes 

 which are usually placed upon movable peduncles ■ while the anterior 

 part of the body is covered with a " carapace " or shield, which 

 covers the head and the anterior thoracic segments at any rate, and 

 often protects the entire cephalothorax. The body consists of nine- 

 teen undoubted segments, of which thirteen belong to the cephalo- 

 thorax and six to the abdomen. If the ocular ring and the telson 

 be counted as segments, there are then twenty-one segments alto- 

 gether. All of the Thoracostraca except certain of the Shrimps 

 (PenceidcB) pass through " Zoea " stages in their development. The 

 division Thoracostraca comprises the four orders of the Cwnacea, 

 the Schizopoda, the Stomatopoda, and the Decapoda, of which the 

 last includes the most highly organised and familiar examples of 

 the class Crustacea. Of these orders, the first two have no direct 

 fossil representatives, though we may consider in connection with 

 the Schizopods certain Palaeozoic Crustaceans, which are possibly 

 ancestral types of the order. The Stomatopods are represented by 

 few fossil forms, beginning, perhaps, in the Carboniferous rocks. 

 On the other hand, there are numerous known fossil forms of the 



