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and most of the males known have 3-jointed antennule, distinct antennz, distinct and, in 
the males, often very considerable trunk-legs as well as caudal stylets, whereas a few species, 
through a reduction of these parts, form a transition to the genera of the last group, having 
antennule which are 2-jointed or quite indistinctly articulated, and no antenne, trunk- 
legs or caudal stylets in either of the two sexes. Another slight difference between the two 
groups may still be mentioned, namely, that in the males the maxillipeds, especially their 
distal part, is stouter and more normally developed in the first than in the second group. 
Homoeoscelis does not deviate very much from Spheronella, though it differs distinctly 
in having trunk-legs and caudal stylets, which are similarly shaped in the same individual, as 
well as in the two sexes, and whose form differs very much from that in the female of 
Spheronella; finally, the larve of this genus differ from all others in their very long antenne 
and in the great distance between the maxille and the maxillipeds. It is also a character 
of this genus that its species occur in the branchial cavity of Cumacea. In Choniostoma, 
unfortunately, the male is unknown; the female only differs from Spheronella in haying 
rudimentary maxillipeds; however, this feature, as well as the fact that its species live in 
the branchial cavity of Hippolyte, appear to me sufficient to maintain the genus. Mysidion 
and Aspidoecia are distinguished most decidedly by differences in both sexes and in the 
mode of living, which it is hardly necessary to mention in detail, and Aspidoecia is the 
most reduced of all forms of the family, both sexes having 1-jointed antennule, no antenne, 
very small mavxillulae without additional branch, and, as a matter of course, no trunk-legs 
or caudal stylets; moreover, the mavxillipeds are entirely wanting in the female, and their 
distal part is greatly reduced in the male. 
No less than thirty-four of the here described species are referred to the genus 
Spheronella, and these species differ very much from each other in several respects which, 
at least apparently, are of considerable importance: 1) Antennule mostly 3-jointed in both 
sexes, sometimes shorter and either 2-jointed or with indistinct articulation. 2) Antenne closely 
similar in both sexes, generally pretty well developed, in some species rudimentary, in others 
wanting. 3) Maxillule almost alike in both sexes, generally with an additional branch, some- 
times without it. 4) Maxille rudimentary in the female of S. marginata, well-developed in all 
other forms. 5) Trunk-legs and caudal stylets are good-sized in the males of most species, but 
are wanting in a few; these appendages are found in most of the females, though they are very 
small; they are wanting in some forms, and it may be said that where they are wanting in the 
male, they are also wanting in the female of the same species, though the reverse is not always 
the case. 6) The peculiar attachment of the female in S. paradoxa. — In spite of these salient 
differences I have not ventured to divide the genus into two or more genera, as I have been 
unable to discover any feature of sufficiently decisive importance. For it is easy enough to say, as 
many authors do, that if a species (as e. g. S. paradoxa) presents some striking characteristic, 
it must be set apart as the type of a new genus, but frequently we have no guarantee that 
such a feature is really of sufficient importance. We meet a similar difficulty where several 
