44G 



GEOLOGICAL SURVEY OF THE TERRITORIES. 



(in Peltocaris miDute) spines. In tlie presence of the telson in tlie typi- 

 cal fossil genus CeraMocaris we certainly have an important character 

 separating the type with its allies from ISTebalia, and allying them to 

 the Decax)ods ; and thus in the provisional synopsis of the order pre- 

 sented farther on, we have placed the fossil forms in a separate suborder 

 from the Nebaliadce. 



CLASSIFICATION OF THE SUBCLASSES AND ORDERS OF CRUSTACEA. 



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 lloeari 





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 1 





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Neocarida. 





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Pal^ocarida. 



Crustacea. 



While the posterior edges of the abdominal segments in Hymenocaris 

 appear to be spined as in Nebalia, there are some characteristics of im- 

 portance in the fossil forms which deserve mention; these are the sculpt- 

 ured carapace, especially of Dictyocaris, in which the surface is reticu- 

 lated.' Moreover the size of these genera was enormous, but if we, as 

 we seem to be warranted in doing, regard Nebalia as a survivor and 

 decrepit or old-age type of the order, which has lost the ornamentation 

 of the integument, the size, and the telson even, being dwarfed, smooth- 

 skinned, and in general very simple compared with the forms which 

 existed at the time when the type culminated and before it began to 

 die out, we may have an explanation of the greater simplicity of the 

 carapace and abdomen of Nebalia, as compared with its paleozoic an- 

 cestors. 



From our total lack of any knowledge of the nature of the limbs of 



'It should here be remarked that while the carapace of Nebalia is smooth, upon 

 making a section of it a reticulated structure is plainly seen in ^he parenchyma or soft 

 parts of the shell, but it is entirely too minute to be perceptible m the shell even under 

 liigh powers. Tlais structure may be comparable with that of Dictyocaris, especially 

 as Salter remarks (Ann. and Mag. Nat. Hist., 1866, p. 161): " The entire surface of the 

 carapace is marked with hexagonal reticulations one-thirtieth of a line in diameter, 

 of which the arete are convex and the bounding lines sunk on the exterior aspect. 

 This would, I think, indicate the ornament to be counected with the structure of the 

 carapace rather than to be a mere external sculpturing. As no films can be obtained 

 thick enough to furnish a section for microscopic examination, the point cannot be 

 ascertained." 



