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 SEROLIDZ; 2. C. ERRANS 
or AXcID#; and 3. C. PARASITES or CYMOTHOIDA. 
The first of these tribes consists of the singular genus 
Serolis, 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 antennz, 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 Avcipz#, 
* The Cymothoade 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 Sphawromide and the 
genus Limnoria. 
VOL. Il. 2 
