J. D. Dana on the Classification of Crustacea. 9 
croura. 'lhere 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 minth step is seen 1n the Squilloidea, whose relaxation 
of system and elongation in the cephalie part, as well as abdo- 
men are remarkable. 
The continuation of the line of degradation represented in 
the AÁnomoura, is not to be found, as we have remarked, among 
the typieal Macroura. But the structure of the Paguri may be 
traced into the aberrant Macroura, called  7Aalassimidea ; and 
thence, both in the abdomen, the legs, and the branchie, 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 Ànomoura and higher species. 
In our review, thus far, we recognise one only of the primary 
types of structure among Crustacea. 'lhis primary type 1s char- 
acterized by having nne normal annuli or segments devoted to 
the senses and mouth, that is, to the cephahce portion of the 
body. It includes &wo, or, we perhaps may say, three secondary 
types. 'The first of these secondary types is the Brachyural; it 
has the antennz small,the inner pair in fossettes, the abdomen 
without appendages. In the other type (or other two, if so con- 
sidered), the antenn: are elongated, and both pairs free, the ab- 
domen is elongated, and furnished with a series of appendages. 
"his, 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 'T'halassinidea. 
Hach secondary type embraces types of more subordinate 
character, which it is unnecessary here to dwell upon. 
There 1s à 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 cephalie annuli; but itis only a modifieation 
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. 
