1852.] . 11 



These subtribes may be divided into families. 



Subtribe 1. Tkalassiiiidea. This section, as Milne Edwards observes, includes 

 two strongly marked divisions; ow, with only the ordinary thoracic branchiae, 

 and a second with the addition oi abdominal branchial appendages, as in the Squil- 

 lidae. The former we name the Thalassinidea Eubranchiata. the latter, the 

 Thalassinidea Anomobrayickiata. The first group embraces three families, dif- 

 fering strikingly in outer maxillipeds and abdomen, as explained beyond. The 

 second contains only two genera, Callianideai Edw., and IscRa^ Guerin the last 

 name was changed by Edwards to CalUanisea ; but as this word is so near Callia- 

 nassa and Callianidea, a contraction to Callisea would be preferable. 



Subtribe 2. Astacidea. In this subtribe, we adopt De Haan's sections, except 

 that we exclude the Megalopidea, and we do not associate the Thalassinidea 

 with the Astacidea. The sections or families are Scyllaridce, Fali?iundeB, 

 Eryonidce and Astacida. 



Leach in 1819 divided the old genus Astacus, naming the marine species 

 (Homarus Edw.) Astacus, and the fresh water (Astacus, Edw.) Potamobius, 

 Edwards' division, of like character, now generally accepted, was not published 

 till 1837. Leach hence has the priority. But according to Leach, the name 

 Astacus is appropriated, not to the typical part of the group, that including the 

 Astaczis fluviatilis of old authors, or Cancer Astacus of Linnaeus, and which em- 

 braces at the present time numerous species, but to that including the Cancer 

 Gammarzcs of Linnaeus, still but a small group. There is hence much objection 

 to the names of Leach, and moreover much confusion would now ensue from 

 their adoption. There seems therefore to be sufficient reason for rejecting them, 

 if it be of no weight that they have remained for 30 years unrecognised by 

 British authors. They are adopted in the Catalogue of British Crustacea of the 

 British Museum, published in 1850, but not in the general catalogue of 1847. 



Subtribe 3. Caridea. In arranging the Caridea into groups, much stress is 

 usually laid upon external form and length of beak. The unimportance of these 

 characters is inferrible from the fact that they involve no essential variations of 

 structure. Moreover, in a single natural group we may find both the long and 

 short beak. In the Crangon group, for instance, in which the beak is usually 

 very short and the body depressed, we have a species with the beak and habit of 

 a Hippolyte. 



There are other characters of more fundamental value ; and these have been 

 brought forward by De Haan. The mandibles afford the distinctions alluded to. 

 In one section they are very slender and are bent nearly at a right angle, with- 

 out enlargement at the crown. In another they are very stout, and somewhat 

 bent above with a broad dilated crown. In a third, they are stout, but not bent, 

 and have a dentate summit. In a fourth they have, in addition to a projecting 

 lateral crown, a large summit process, which is often oblong and very prominent. 

 These forms are characteristic of different sections of the Caridea. 



The fact that the mandibles bear a palpus or not is of much less importance ; 

 for the portion of the mandible which is most essential to its functions is the 

 crown. Among the Palaemoninae, there are genera having a mandibular palpus, 

 and others without one; while the two kinds in other respects are remarkably 

 close in their relations. We have found moreover that in this group, the length 

 of the palpus varies with the disjunction of the 2d and 3d flagella of the inner 

 antennae. If these flagella are separate to their bases nearly, (as in Palasmon,) 

 the palpus is long and 3-jointed ; if united for some distance up, the palpus be- 

 comes short and finally only 2-jointed (Palaemonella;) if united nearly or quite 

 to their summits, there is no palpus.* 



In the arrangement of the genera into families, the fact whether the 1st or 2d 



* In our genus Palsemonella, the palpus of the mandible is 2-jointed, and in ./fn- 

 chistia, which is closeiy like Palaemon in habit in some of its species, there is no 

 palpus, as in the Pontoniae ; and thus the transition to the Pontoniae from Palaemon 

 is exceedingly gradual. Harpilius and CEdipus (Pontoniae of authors) fill up the in- 

 terval between Anchistia and the true Pontoniae. They are all similar in having the 

 2d pair of legs largest, and in other prominent characteristics. 



