]. '08. 127 



Family PALAEMONWAE. 



The three British and Iriah genera may be distinguished 

 thus : — 



I. Rostrum very short, unarmed; antennuies 

 biramous — the outer flagelhim not spUt ; 

 second pair of pereiopods assymmetrical ; [no 

 palp on mandible]. Living in sponges, . Tijpton. 



II. Kostrum well developed, bearing teeth above and 

 below ; antennules trii-amoiis — the outer tlagel- 

 lum split ; second pair of pereiopods symme- 

 trical. Free living. 



A. Mandible with a two or tliree jointed palp, 



. Leander (pp. 128, 129). 



B. Mandible without palp. . . Palaemonetes (p. 129). 



The genus Typton, Costa, is very closely related to Pontonia, 

 Latreille ; many authors consider that the characters of these 

 and of a few other allied genera entitle them to rank as a 

 sepajate family, the Pontoniidae. 



Of Typton, one species only, T. spongicola} Costa, has been 

 found witiiin the British area, but it has not so far been 

 observed in Irish v/aters. It has been recorded three times 

 from the coasts of Devon and Cornwall (v. Xorman and 

 Scott, 1906), where it was found living within the sponges 

 Desmacidon and Homoeodictya. 



Of the genera Leander and Palaemonetes, four species, all 

 of which were known to Bell, are found off the British coasts. 

 Of these Leander sqiuJla is certainly one of the commonest, 

 yet it was not hitherto known that this abundant form differed 

 from the two allied British species, L. serratus and L. ad- 

 spersus in such an important character as the number of joints 

 in the mandibular palp. Recently Dr. Caiman, having had 

 occasion to examine these species, discovered that in L. squilla 

 only two segments are to be found in the palp, whereas three 

 are present in L. serratus and adspersus. This most interest- 

 ing information Dr. Caiman has very kindly placed at my 

 disposal — as may be seen from PL XX, figs. 2, a-c, I am able 

 to testify to the accuracy of his observation. 



On the following pages the principal characters of the 

 British species of Leander and Palaemonetes are summarised 

 in the form of a table. 



1 For synonymy consult Norman, 1868. 



