SURFACE FAUNA OF THE GULF STREAM. 13 



As will be seen from the acconipanving description, the other differences 

 to be noted between the Florida and the Mediterranean species are : the 

 numerous and close corrugations of the lower surfoce of the disk ; its great 

 roughness on the upper side ; the compai'atively greater length of the 

 primary polj'pites ; the smaller size of the proliferous poh'pites. The shape 

 of the former is very different from those of the Mediterranean species, ending 

 in three to four large knobs, with three longitudinal rows of smaller ten- 

 tacular kuo1>s. — six in two of the rows, and from eight to nine in the 

 central row (PL X, Figs. 3, 5, 7). 



As in the Mediterranean species, the disk is circular, slightly cup-shaped, 

 convex above, made up of two disks, thickest near the outer edges, and 

 united together by a series of circular walls entirely separated from one 

 another (PL XT, Figs. 7-13). The circular chambers thus formed ojien out- 

 wardlj* by small elliptical openings giving air free access to them (PI. XI, 

 Figs. 7-9, 12, o). These chambers are filled with air (gas ?), giving to the 

 Porpita its great buoyancy. The enclosed air (gas ?), shining through the 

 float and the thin mantle which covers the disk, gives to its upper side a 

 strong silvery lustre. The upper side of the disk is but slightly corrugated, 

 with radial depressions, the stigmata being placed in the centre of low pro- 

 jections, forming an irregular elevation in the lines of corrugations of the 

 disk (PL XI, Fig. 7). There is a large central chamber, with an irregular 

 ring of eight smaller ones (PL IX, Figs. 1-4), corresjDonding to the eight 

 first-formed triangular chambers, placed round the central chamber. There 

 does not seem to be beyond that first row any regular arrangement in the 

 stigmata, or any order in their appearance or number. They vary greatly in 

 position and in number in specimens of the same size. The stigmata are 

 more numerous, as well as larger, near the edge of the disk, as has already 

 been noticed by Kolliker. 



The ribbed structure of the lower side of the disk does not seem to have been 

 observed. The lower floor of the radiating chambers extends into deep longi- 

 tudinal corrugations (PL XI, Figs. 5, 8-11, 13, 14), forming immense pouches, 

 as it were ; so that when seen endwise they present the appearance of high- 

 pointed bags, with rounded tips, and deep spaces between them (PL XI, 

 Fig.s. 13, 14). A transverse (PL XI, Figs. 8, 9) and a longitudinal view 

 (PL XI, Figs. 10, 11), will give a better idea of their peculiar structure than 

 any lengthy description. To the under side of these pouches are attached the 

 tubiiles (PL XI, Figs. 1, 2, 3, 5, 8, 9), they are irregularly placed in single rows 



