The Penis-hone^ or '■'' Baculum^^'' in certain Squirrels. 383 

 Literature cited. 



[i] Andrews, C. "W. ' A Monograph of Christmas Island.' Published 



bv the British Museum, 1900. [Note on Cardisoma carnifex 



(C. hirtipes), p. 164.] 

 [2] Ba"slis, H. A. " Oiigochaeta " : British Antarctic ('Terra Nova') 



Exp. 1910. Zoology, vol. ii. no. 2, pp. 13-18. Published by the 



British Museum, 1915. 



Other Papers consulted. 



[3] RiCHABD, J. " Essai sur les Parasites et les Oommensaux des Crua- 



taces." Arch. Parasitol. toui. ii. pp. .548-59.5. Paris, 1899. 

 [4] Wilson, C. B. " Crustacean Parasites of AN'est Indian Fishes and 



Laud Crabs." Proc. U.S. Nat. Mus. vol. xliv. (1913) pp. 189-277. 



\_Cancrinrola, a new Genus of Copepoda, Parasitic in Cardisoma 



yuanhumi, gills, p. 264.] 



XXXIV. — The Penis-bone, or ^^ Baculum," as a Guide to 

 the Classification of certain Squirrels. By Oldfield 

 Thomas. 



(Published by permission of the Trustees of the British Museum.) 



There has always appeared to be something wrong witli the 

 inclusion of the Oriental squirrels in the same genus as 

 Sciurus vulgaris, althougli when classifying the group some 

 years ago I was unable to find any material differences in 

 their skulls and teeth. 



ISIovv, however, I have found a character by which such 

 squirrels as are still put in Sctunis may be sorted into 

 several groups, each sharply defined from the others. 



This is in the structure of the os penis, which shows very 

 striking differences between the various groups of species, 

 and may evidently be of great service in classifying the 

 members of this difficult family. For many years I have 



of this species. The specimens differ in the form of the carapace and 

 third maxillipeds from G. maljnlensis as described by Faxon (Mem. Mus, 

 Comp. Zool. xviii. 1895, p. 28); in the proportions of the carapace and 

 legs from G. digueti, Bouvier, as redescribed by Rathbun (U.S. Dep, 

 Agric. N. Amer. Fauna, no. 14, 1899, p. 73) ; and in having .six spine- 

 rows on the dactyli, from G, lateralis (Freminville). I am not aware 

 that anyone has attempted a detailed criticism of Ortmann's opinion 

 (Zool. Jahrb. Abth. f. Syst. x. 1897, p. 337) that all the American forms 

 belonging to this genua can be referred to a single species, G. ruricola 

 (Linn.)." 



