6 BRITISH FOSSIL CRUSTACEA. 
We have seen that the generally accepted type-number of body-rings, or somites, is 
twenty-one; but in the fossil 7riodcéa," and the recent Phyllopoda and Branchiopoda, 
we have instances in which a larger number of segments-occur. On the other hand, we 
occasionally meet with forms, both recent and fossil, in which one or more segments are 
never developed; but in general their apparent absence is due to their coalescence, and 
we shall frequently find indications of this if we bear in mind the theory of Oken, that 
each pair of appendages indicate a separate segment. | 
In the illustrations of recent Crustacea which we have given on Plate IX we have 
numbered the somites of figs. 1, 2, 3, 4, and 5, so as to show at a glance how many 
segments are united to form the head in each, as indicated by the figure placed upon it. 
The restored figures of Pferygotus anglicus, Ag., Plate VIII, are also similarly numbered. 
“In the embryo these segments are formed in succession from before backwards, so 
that, when their evolution is checked, the later, rather than the earlier rings, are those that 
are wanting ; and, in fact, it is generally easy to see in those specimens of full-grown crus- 
taceous animals whose bodies present fewer than twenty-one segments that the anomaly 
depends on the absence of a certain number of the most posterior rings of the body.” 
Just as we find a typical number of twenty-one body-segments to prevail among the 
Crustacea, so also in the appendages, the type number of joints is seven, any departure 
from which is disguised by fusion of one or more joints together, the obsolete condition of 
others, or the depauperization of the limb into numerous articulz. (C. Spence Bate.) 
The walking leg of a Decapod Crustacean (see woodcut, fig. 1), or the maxillipede * of 
Pterygotus anglicus (see woodcut, fig. 2), will serve as illustrations of limbs having the 
type-number of joints, which we will designate as follows : 
Coxa : ; o I 
Basos 2 
Ischium 3 
eros 3 ; 4 
Caurpus 5 
Propodos . : 5G 
Dactylus 7 
Fic. 1. Walking leg of Fic. 2. Jaw-foot of 
Decapod, Pterygotus.8 
' See Mr. Salter’s Monographs on the Jrilobita, Fal. Soc., 1864-6. 
2 Milne-Edwards, ‘Todd’s Cycl. Anat.,’ Article Crustacea, p. 753. 
5 ‘Ketognath’ of Prof. Huxley, ‘ Geol. Surv.,’ Mon. I, 1859, p. 19. 
