A REVISION OF THE BRITISH IDOTEID^E. 



729 



From the Mediterranean I have the same species, but in all the specimens the 

 tridentate terminal segment of the metasome is less pronounced, and the mesosome 

 in both sexes is comparatively wider. 



In examples of /. linearis (Pennant) received from Plymouth, eight specimens 

 all show a depressed area on the ventral side of the fifth peduncular joint of the 

 antennae, and in one example of the same species, received from the Bay of Nigg, 

 Aberdeen, the left antenna has six peduncular joints. 



There appears to be a considerable amount of variation in the size of the different 

 species according to locality. Of the species treated of in this paper I give below a 

 list, and the measurements given by different authors. The first line of figures refers 

 to the length of the male, and the second to that of the female, where two are quoted. 



Species. 



Authority. 



Batb and 

 Westwood. 



03 



a 



n 



■< 



M 



CO 



m 



o 

 m 



a 



-X 



a 



o 



EH 

 EH 



O 

 O 



m 



<( 



03 



« 



H 

 H 

 Eh 

 < 



m 



03 



H 



W 



Ph 

 W 

 EH 



m 



k4 



H 



03 

 03 



1— 1 



H 



g 



ti 

 hi 



o 

 O 



1. /. baltica (Pallas) .... 



in. 



mm. 

 30 



mm. 

 30-38 



mm. 

 35 



mm. 



20 



mm. 



mm. 

 35 



mm. 



mm. 



mm. 

 35 



2. 1. pelagica, Leach 



i 



2 





20 



13 







13 







20 

 13-14 



3. J. neglecta, G. 0. Sars . 









9 

 25 





25-33 



9 

 25 







9-10 

 27-5 



i. I. emarginata (Fabr.) . 



n 



35 





16 

 30 







16 

 30 







16-17 



30 



5. /. granulosa, Rathke 







25 



18 

 15 







18 

 15 







20 

 19-21 



6. /. viridis (Slabber) 









11 



12 







11 

 12 







16 

 12 



7. /. metallica, Bosc 





17 



28 



10 



... 



18 





10 



18 



18-31 





10 

 23 



8. /. linearis (Pennant) 



H 



38 



22 









35 



18-24 





18 

 28-38 



9. Zenobiana prismatica (Risso) 





15 









... 



12 





10-13 



25 

 135 



10. Synisoma lancifer (Leach) 















20 







22-5 



11. Synisoma acuminata (Leach) . 





25 











15 









VI. External Structure. 



The form and structure of the exoskeleton of this family have only been very 

 imperfectly described and figured. The differences in a given species in the descrip- 

 tions and figures of different authors are difficult to account for. Thus in such a 

 cosmopolitan form as Idotea baltica (Pallas), Sars (64) states that there are sixteen 



TRANS. ROY. SOC. EDIN., VOL. LI, PART III (NO. 23). 107 



