/. D. Dana on the Classification of Crustacea. 21 



croura. There is not merely a relaxing of the centralization ; 

 but the forces are so weakened as not to succeed in finishing out 

 the members in the system of structure to which they pertain. 

 The species consequently are not modifications upon the level 

 of the Macroural type, nor upon a distinct level or distinct type ; 

 but simply imperfect developments of the Macroural structure 

 below the true level of that type. They bear nearly the same 

 relation to the Macroura, that the Anomoura bear to the Brachy- 

 ura. The ninth step is seen in the Squilloidea, whose relaxation 

 of system and elongation in the cephalic part, as well as abdo- 

 men are remarkable. 



The continuation of the line of degradation represented in 

 the Anomoura, is not to be found, as we have remarked, among 

 the typical Macroura. But the structure of the Paguri may be 

 traced into the aberrant Macroura, called Thalassinidea ; and 

 thence, both in the abdomen, the legs, and the branchiae, we ob- 

 serve a transition to the Squilloids, one division of the Anomo- 

 branchiates. If then, we were to trace out the lines of affinity 

 in the species, it would be from the Mysis group to the typical 

 Macroura, and from the Squilla group to the Thalassinidea, as 

 elsewhere explained. From the latter, the lines lead mainly to 

 the Anomoura and higher species. 



In our review, thus far, we recognise one only of the primary 

 types of structure among Crustacea. This primary type is char- 

 acterized by having nine normal annuli or segments devoted to 

 the senses and mouth, that is, to the cephalic portion of the 

 body. It includes two, or, we perhaps may say, three secondary 

 types. The first of these secondary types is the Brachyural ; it 

 has the antennae small, the inner pair in fossettes, the abdomen 

 without appendages. In the other type (or other two, if so con- 

 sidered), the antennas are elongated, and both pairs free, the ab- 

 domen is elongated, and furnished with a series of appendages. 

 This, the second type, is the Macroural ; or, if we assume that 

 it embraces two distinct types (a second and third), the two cor- 

 respond to the typical Macroura and the Thalassinidea. 



Each secondary type embraces types of more subordinate 

 character, which it is unnecessary here to dwell upon. 



There is a tendency in the lowest Macroural species to a trans- 

 fer of the two posterior mouth appendages to the foot series, so as 

 to leave but seven cephalic annuli ; but it is only a modification 

 of the primary type, as the species have every mark of being de- 

 graded or imperfect forms, and are not examples of a new type. 



In this primary type, the species vary in length from half an 

 inch to twenty inches. Two inches may be set down as the av- 

 erage length and breadth for the Brachyura ; while three inches 

 is the average length of the Macroura, the average breadth being- 

 half an inch or less. 



