420 PROCEEDINGS OF THE NATIONAL MUSEUM. vol.39. 



Just as the "annulus ventralis" of the female and the speciaHzed 

 pleopods of the male Cambarus were well known as specific characters 

 before it was loiown that both these sets of organs were essential as 

 sperm receptacles and organs to fill the receptacles, so also in the 

 prawns of the family Peneidse it was well known that the females 

 possessed remarkable structures, called the thelyca, and the males 

 peculiar united appendages, the petasma, before it was known that 

 here also we have useful receptacles and transferers of sperni. 



The following description will show what the thelycum is in three 

 of these prawns, with reference to its use as a sperm receptacle. 



In Peneus setiferus (Linnaeus), P. hrasiliensis (Latreille), and 

 Parapenseus constrictus (Stimpson) the thelycum is found to be a more 

 or less simple cavity on the ventral surface of the thorax between the 

 fourth and the fifth pairs of legs, made by anterior and posterior 

 scales or elevations of the shell that inclose the sperm-containing 

 space. The posterior outgrowth from the sternum of the fifth legs 

 is more or less divided into right and left lobes, and it grows forward 

 to overlap the anterior outgrowth which extends back from the 

 sternal region between the fourth legs. 



In specimens of Peneus setiferus obtained in March at New Orleans, 

 Louisiana, when small, as well as in large specimens up to 180 mm. in 

 length obtained in April at Hampton, Virginia, and in the Baltimore 

 markets in November, the thelycum is so simple as to suggest imma- 

 turity, yet the presence of mature sperm in the April males along 

 with minute young eggs in the females suggests that the males may 

 have been about ready to transfer the sperm to the thelycum, where 

 it would await the maturing of the eggs. 



Figure 1 shows part of the underside of a female with the tliird, 

 fourSi, and fifth left legs removed from the left side. The oval 

 opening of the oviduct is represented upon the base of the tliird right 

 leg. Between the fifth legs two scales (Sc) with a wide groove 

 between them project forward over a depressed area. These scales 

 may be regarded as arising from a common transverse plate some 

 3 mm. wide. Each scale is some L5 mm. long, and its anterior edge 

 is very abrupt, as seen in the lengthwise section, fig. 2 (*S'c). 



The depressed area anterior to the scales is partly overhung by a 

 wide shelf, figs. 1 and 2 (S), wliich is somewhat bilobed and has its 

 posterior overhanging part decidedly bent toward the general shell 

 of the sternal region, so that there is formed a concealed recess in 

 which dirt frequently collects. Several specimens have^ dark, 

 necrotic areas on the shelf, as if the results of injury (possibly in 

 conjugation?). In the November specimens curious remains of 

 minute organisms, apparently stalked protozoa, are found attached 

 to the shell under the above-mentioned shelf. The shelf is depressed 

 ridit and left, so that its middle stands up as a rounded boss, fig. 1 (S). 



