384 Mr. O. Thomas on the Penis-bone, or 



been collecting materials for the study of this bone, and hope 

 later to be able to give a general paper upon it, but, as a 

 preliminary, it seems advisable to publish a note on certain 

 cases affecting nomenclature and generic position. 



Since every other bone of the skeleton has a name of its 

 own, not merely the " bone of the leg " or " bone ot the 

 head,^^ it appears to me convenient to have a special term 

 for tiie bone of the penis, and I therefore propose to call it 

 the baculum, meaning a little stick. 



Now the baculum of Sciurus vulgaris^ the type of the 

 genus, is a very characteristic bone, like a small spatula, or 

 still more like a half-closed human right hand, the shaft 

 forming the forearm, the blade of the spatula the hollowed 

 ])alm, and a small pointed projection on the right side corre- 

 t^ponding to an outstretched thumb. 



Ot this type, and agreeing with it exceedingly closely, are 

 the bacuhi of the other Palsearctic species, S. pei sicus t^wd 

 S. H.i, of the whole ot the American species, so tar as 1 have 

 been able to examine them, and, remarkable to say, of the 

 Bornean Reithrosciurus macrotis. 



But all the Indian and Malayan species hitherto referred 

 to Sciurus have bacula totally different from that of true 

 Sciurus, and themselves divisible into two types, though with 

 an essential community between the two. 



For in all the baculum consists of two parts, a shaft or 

 capulus of varying length and a separate sharp blade or 

 lamina attached to the shaft by ligament and slightly 

 movable upon it. The lamina has a concave base, which 

 articulates with the rounded surface of the shaft, and allows 

 a certain amount of lateral play. 



In position in the penis the blade points to the right, its 

 edge outwards. This edge is very sharp indeed, is practi- 

 cally uncovered by tissue of any sort, and seems to be for 

 the purpose of enlarging the female opening by a clean 

 knife-cut into the tissues. A careful study ot the soft 

 anatomy of the female will be needed before the exact 

 objects and methods of this remarkable structure can be 

 understood. It is possible that the little, sharp, thumb-like 

 projection on the baculum of typical Sciurus has a somewhat 

 similar function. 



As already noted, the compound bacula are of two types, 

 respectively more and less specialized. 



The less specialized consists of a long, slender, slightly 

 curved shaft, with a narrow blade set on the side ot it, in the 

 concavity of its general curvature. The blade is attached 

 nearly tiiroughout its length, and its greatest breadth is only 



