1. '08. 130 



Certain genera of Palaemonidae such as Palaernoji, Palae- 

 monella, Amphipalaenion, Palaemoneies and Leander form 

 such a lioniogeneous group that few reliable characters are 

 available for their determination, and consequently the num- 

 ber of joints of which the mandibular palp is composed, or its 

 total suppression, become indications of primary importance. 

 The question therefore arises whether squilla and serratus 

 should not properly be placed in different genera, even though 

 the only character which can be found to justify such a view 

 is afforded by the segmentation of the mandibular palp. As a 

 precedent for such a course the genera Palaemonella and 

 Palaemon might be cited, in which the chief and perhaps the 

 only differential characteristic lies in this very detail of a two 

 or three jointed palp. Nevertheless it seems best for the pre- 

 sent to retain the three species under the genus Leander'^. 

 It is difficult to estimate the value of a single character of 

 such a nature as this without a thorough investigation of all 

 the genera of the family, and until this much needed work is 

 undertaken the matter is best left untouched. 



Bell distinguished the four British species from one another 

 by characters drawn almost solely from the rostral dentition, 

 and although it is true that all four can, as a rule, be deter- 

 mined by this feature alone, yet the considerable variation 

 which exists renders it in practice less useful than others. 

 From the table on the preceding pages it will be noticed that 

 apart from the important question of the mandibular palps all 

 four species can be determined by the characters afforded by 

 the second pair of pereiopods. 



Genus Leander, Desmarest. 

 Palaemon, Bell, 1853. 



Leander serratus (Pennant) 

 PL XX, figs. l,a-e. 

 Palaemon serratus, Bell, 1853, fig., p. 302. 



The colouring in life of this and of the three succeeding 

 species is extremely variable and doubtless depends largely on 

 environment. The species are littoral in habit and have only 

 rarely been found during the investigations of the Helga ; it 

 is therefore not possible to discuss their colouration in any 

 adequate manner. 



1 The type of the genus Leander is L. errafdcus, Desmarest, which 

 Speiice Bate (1888) quotes as a synonym of L. natator (Mihie-Edwards). 

 Stimpson (1860) cites L. natator as the type species, and Dr. Caiman in- 

 forms me that specimens in the British Museum labelled as this specie? 

 possess a three-jointed palp on the mandible. 



