50 CRUSTACEA. 



We here give only a summary of the prominent characteristics, in 

 order to illustrate thereby the gradations into this type from the 

 Brachyural. 



From the high rank of the typical Maioidea, the first point of degra- 

 dation seen is in the Parthenopinea. They are the Cancroid forms of 

 this group, having the short epistome of the Cancroidea, and the base 

 of the outer antennae usually bounded exteriorly by a suture, and 

 commonly a rather broad front, though still rostrate. 



The second step in degradation is to the Oncininea, the outer an- 

 tennae being here wholly free and cylindrical. Moreover the two 

 posterior pairs of legs are prehensile. The other characters pertain 

 to the Maioid type, and are in accordance with the typical Brachyura. 



The grade next lower carries the series below the true Brachyural 

 level. Either the branchiae are more numerous; or the outer antennae 

 are posterior to the eyes; or the inner antennae have no fossettes; and 

 in connexion with one or the other of these marks of degradation, the 

 vulvae are peculiar in being situated in the basal joint of the third pair 

 of legs, as in the Macroura, instead of in the sternal plate, as in the 

 Brachyura ; and the abdomen begins to show some traces of increase, 

 either in its appendages, or size, or both. Besides, the posterior legs 

 are more or less prehensile, as in Oncinopus, and also much smaller 

 than the others, this smaller size, both here, and where it occurs in 

 the Macroura, being a mark of low grade. Internally, the sella tur- 

 cica, median apodeme, and female copulative pouch, are wanting. 



This degradation is seen in Latreillia, an Inachoid form; the poste- 

 rior legs being shorter and prehensile the anterior antennae without 

 fossettes the posterior antennae arising from behind the eyes the 

 vulvae in the base of the third pair of legs the sella turcica and 

 median apodeme wanting. The species, however, have the Brachyural 

 number of branchiae, and the habit of a Leptopodia. 



It is observed again in Dromia, a Trichia-like form. The genus 

 Trichia is of the Parthenopinea group, and is Libinioid in aspect; it is 

 a transition genus between the Parthenopinea and the Dromioids. 

 Here the antennae are as in the Parthenopinea; but the four posterior 

 legs are prehensile, the branchiae abnormal (fourteen) in number, the 

 vulvae, sella turcica, and median apodeme as in Latreillia. The abdo- 

 men in both these genera is small, yet in the last, there are traces of 

 a transition character. 



A still lower degradation of the Maioid type brings us to Lithodes 



