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BRITISH FOSSIL CRUSTACEA. 



We have seen that the generally accepted type-number of body-rings, or somites, is 

 twenty-one ; but in the fossil Trilobita} 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 Pterygoids anglicm, 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." 2 



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 articuli. (C. Spence Bate.) 



The walking leg of a Decapod Crustacean (see woodcut, fig. 1), or the maxillipede 3 of 

 Pterygotus angliciis (see woodcut, fig. 2), will serve as illustrations of limbs having the 

 type-number. of joints, which we will designate as follows: 



Coxa 



Basos 



Ischium 



Meros 



Carpus 



Propodos 



Dactylus 



1 



2 

 3 

 4 

 5 



6 

 7 



Fig. 1. Walking leg of 

 Decapod. 



Fig. 2. Jaw-foot of 

 Pteryyotus? 



1 See Mr. Salter's Monographs on the Trilobita, Fal. Soc, 1864-6. 



2 Milne-Edwards, 'Todd's Cycl. Anat.,' Article Crustacea, p. 753. 



s 'Ectognath' of Prof. Huxley, ' Geol. Surv.,' Mon. I, 1859, p. 19. 



