CYMOTHOID^. 273 



The second subtribe of the Parasitical Normal group 

 of the Isopoda contains all those species in which the 

 sexes are not materially unlike each other, the males 

 being generally nearly as large as, or larger than, the 

 females, with the dermal tissue hard and crustaceous, 

 the pereiopoda well developed, the oral organs formed 

 for mastication, the mandibles being horny, having the 

 terminal half narrowed and curved inwardly, with the 

 apex formed for gnawing, the base also furnished with a 

 triarticulate appendage. The pleon is terminated by a 

 large flat crustaceous plate representing the sixth seg- 

 ment, bearing near its base at each side a pair of elongate 

 flattened lobes (uropoda) arising from a dilated base. 



These animals compose the family of Cymothoadiens 

 of Milne Edwards,* which he has divided into three 

 tribes corresponding with our families — namely, 1. Cymo- 

 thoadiens RAVISSEURS or SeROLID^ ; 2. C. ERRANS 

 or ^GiD^ ; and 3. C. parasites or Cymothoid^. 



The first of these tribes consists of the singular genus 

 Sej'olis, the species of which are thought by many to 

 offer a very strong resemblance to the Trilobites. They 

 are all natives of the Ocean surrounding South America, 

 and are distinguished by their flat circular form with 

 moderately long antennas, and with the first or the first 

 and second pairs of legs strong and cheliferous, whilst 

 the remainder are slender and fitted for walking. 



The second tribe corresponds with our family JE^gidm, 



* The Cymothoadaa of Dr. Leach's article in the " Dictionnaire des Sciences 

 naturelles" was still more extensive, as he introduced into it not only the 

 groups admitted by M. Milne Edwards, but also our Sphceromidce and the 

 genus Limnoria. 



VOL. II. T 



