THE PERIPROCT. • 173 



six cases in 2, III, II; in three cases in 2, 3, III, and II; in four cases in 2, 3, II (text-fig. 

 148, p. 134). In seven cases madreporic pores occur in genitals 1, 2, 3, and ocular III; in four 

 in 1, 2, 3, and II; and in eleven in 1, 2, 3, and II, III (Plate 5, fig. 14). The most extreme range 

 observed was one specimen in which the madreporic pores extended from genital 2 to genitals 

 1, 3, 4, and oculars III, IV, V. A similar range of variation in the distributipn of madreporic 

 pores and in about the same proportions has been found in other species, especially in the 

 families Echinidae and Strongylocentrotidae. In the young, madreporic pores very rarely 

 extend beyond genital 2, whereas they frequently do in the adult. This change evidently is 

 taken on as the animal increases in size to the adult condition. 



Occasionally madreporic pores invade interambulacrum 2, and in one somewhat distorted 

 Strongylocentrotus drobachiensis this invasion was extensive. All this goes to show that while 

 typically madreporic pores are limited to genital 2, they may extend from this plate to adjacent 

 genitals, oculars, or interambulacrum, but apparently never to plates of the periproct. They 

 seem never to be absent from genital 2 in Recent regular Echini. When madreporic pores 

 extend beyond genital 2, in the great proportion of cases they move to the left of the antero- 

 posterior axis, emphasizing by this a bilateral axis or plane of symmetry through III, 5. 



As seen from these studies, the genital plates have nothing to do with the interambulacrum, 

 which develops on either side of the oculars (p. 62). The genitals typically possess genital 

 pores, and one of them possesses madreporic pores, but both of these structures may pierce 

 other parts of the test. Genital plates may, therefore, be considered as structures of secondary 

 importance, of much less morphological value than are the oculars. 



The Periproct. 



The periproct is an area of considerable interest. Unfortunately in fossils it is rarely 

 preserved, as the plates easily drop out and are frequently wanting, even in museum specimens 

 of Recent Echini. It is worth noting that in Recent Echini periprocts and peristomes may be 

 rendered strong and safely fastened in place by dipping the specimens in dilute shellac dissolved 

 in alcohol, or gelatine dissolved in water. The same treatment is very eiTective in strength- 

 ening the thin tests of delicate Echini ami in keeping the spines firmly in place. 



In Palaeozoic Echini the plates of the periproct are small, angular, and completely fill the 

 area, as in cidarids. In Bothriocidaris archaica there are nine small plates that fill the area 

 dorsal to the genitals (Plate 1, fig. 2). In Hyatlechinus beecheri (Plate 25, fig. 5) a few peri- 

 proctal plates are preserved (others restored in the figure), indicating a periproct essentially 

 as in Cidaris. In Palaeechinus (Plate 31, fig. 4), Maccoya (Plate 34, fig. 6), Lovenechinus 

 (Plate 42, fig. 6), and Melonechinus (Plate 56, fig. 1) the periprocts as far as preserved are made 

 up of many polygonal, rather thick plates filling the area, that are essentially like those of 



