2 BRITISH FOSSIL CRUSTACEA. 
of Great Britain ;’ to this latter work especial reference will frequently be made 
hereafter. 
The Crustacea are included in the sub-kingdom Arricunata, and are characterised 
by a body divided into rings or segments, more or less distinct and moveable, and pro- 
tected by a horny or calcareous exo-skeleton, provided with articulated limbs arranged in 
pairs, usually five® to seven® in number. 
The class is essentially aquatic, breathing by branchiz; and, although some members 
subsist on land, their organs of respiration are true branchie, and quite dissimilar from 
the trachee of insects. 
Before attaining the adult condition the marine species pass through a series of 
embryonic changes, apparently more numerous in the higher forms.* Every part that 
is present in the larva, though not permanent in the individual, is to be found in a per- 
manent condition in one or other form of adult Crustacea of a lower order than that to 
which it belongs.’ 
Even after arriving at the adult state the shelly envelope is not permanently retained, 
but is exuviated as often as the growth of the animal necessitates its enlargement. In 
the perfect Insect, on the contrary, the exo-skeleton is retained, and no increase of growth 
or reproduction of lost appendages takes place in the dago, as among mature Crustacea. 
Previous to describing the order which forms the subject of this Monograph, it will 
be well to speak of the type on which the class is constructed. By the ‘type,’ we under- 
stand that example of any natural group which possesses all the leading characters of that 
eroup. For it must be borne in mind, that every division of animals, whether vertebrate 
or invertebrate, necessarily includes within its limits genera most dissimilar from the type- 
form upon which the class is constructed. No one, for example, would select the 
Ornithorhynchus as a typical mammal, or the dplerye as a typical bird, the tunicated 
Botryllus as a type-mollusk, or the Balanus as a type-crustacean. 
Indeed, the type of any class or order is not to be sought for at the extremity of the 
series, but near the centre. 
Dr. Milne-Edwards°® writes, “The normal number of segments is twenty-one.” . . . 
“By general consent and usage three regions are recognised in the bodies of 
’ “Memoirs of the Geol. Survey of the United Kingdom,’ Monograph I, 1859. ‘‘On the Anatomy 
and Affinities of the Genus Pterygotus,” by Prof. T. H. Huxley, F.R.S., &c., with ‘Descriptions of the 
Species,” by J. W. Salter, F.G.S., A.L.S. 
2 In the Decapoda. 3 In the /sopoda. 
‘ The land and freshwater Decapoda, as well as the sessile-eyed forms, apparently undergo these 
embryonic changes in the ova, the young nearly resembling the parent when excluded from the egg. See 
also Prof. Bell’s “Hist. Brit. Stalk-eyed Crustacea,’ Introduction, pp. xliv—xlvi; and Prof. Owen’s 
Lectures, 1855, pp. 334-342. 
5 Spence Bate, “On the Development of Decapod Crustacea,”’ ‘Phil. Trans.,’ 1858, p. 602. 
6 See article Crustacea, by Dr. Milne-Edwards, in Todd’s ‘Cyclopedia of Anatomy,’ vol. i, 1836, 
p. 704. 
