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PROCEEDINGS OF THE NATIONAL MUSEUM. 



VOL. 39. 



In the males there is neither shelf nor scales, but there is a pro- 

 nounced median keel .along the area that is depressed in the female. 

 vSome faint suggestion of such a keel is seen in the female also. 



If this thelycum is complete in this state, we may suppose the more 

 important part is the overhanging shelf under which the sperm might 

 be deposited, while the scales might have merely the value of hold- 

 fasts for the petasma of the male. However, nothing was found out 

 as to the mode of functioning in this species. 



A second species, Peneus Irasiliensis, has a similar thelycum with 

 spenn within it. 



Fig. 3.— Surface view of thelycum and adjacent parts of thorax of Peneus brasiliensis. 

 rv and V BASES of left fourth and fifth thoracic legs; S, base of shelf-bearing spine; 

 Sc, closed scale. 



In large specimens, 110 mm. long, fig. 3, there is a shelf and two 

 scales to form the thelycum, but the proportions are very different 

 from what they are in P. setiferus. The shelf (S) is largely con- 

 cealed by the scales (Sc) which run forward like two great doors, 

 leaving only the median boss of the shelf showing between the fourth 

 legs, while all the depressed region is overarched and concealed. 

 However, in young specimens 60 mm. long the two scales are very 

 widely separated along the median line, and anteriorly they cover 

 the shelf only at its sides, so that the shelf appears not only as a 

 high boss that runs back as a spine between the scales, but also as 



