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and most of the males known have 3-jointed antennulae, distinct antennae, 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 

 antennulse which are 2-jointed or quite indistinctly articulated, and no antennae, 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 SpJieeronella, 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 

 SpJieeronella; finally, the larvae of this genus differ from all others in their very long antennae 

 and in the great distance between the maxillae 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 SpJieeronella in having 

 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 antennulae, no antennae, 

 very small maxillulae without additional branch, and, as a matter of course, no trunk-legs 

 or caudal stylets; moreover, the maxillipeds 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 

 SpJieeronella, and these species differ very much from each other in several respects which, 

 at least apparently, are of considerable importance: 1) Antennulse mostly 3-jointed in both 

 sexes, sometimes shorter and either 2-jointed or with indistinct articulation. 2) Antennae closely 

 similar in both sexes, generally pretty well developed, in some species rudimentary, in others 

 wanting. 3) Maxillulae almost alike in both sexes, generally with an additional branch, some- 

 times without it. 4) Maxillae 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 



