172 UOHKHT TRACY JACKSON ON ECHINI. 



openings. Otherwise all the pores in genital plates doubtless connected with genital glands, 

 as in Recent Echini with accessory pores. 



In very young Recent Echini, as shown by Love"n (1892), there is a single madreporic 

 opening in genital 2 (Plate 2, fig. 3; Plate 3, fig. 14; text-fig. 131, p. 129). Fine madreporic 

 pores soon appear, and in the adult are usually numerous (Strongylocentrotus, text-figs. 132- 

 139, p. 132). In some types there are few madreporic pores in the adult, a primitive character 

 (Salenia, Plate 4, fig. 1). Usually the madreporite is perforated by many pores, as seen from the 

 interior as well as from the exterior. In the Cidaridae, however (text-figs. 59,60; 70, 71, pp. 

 95, 98), while there are many on the exterior of the plate, there is only a single large madreporic 

 pore on the interior, so that at this part of the plate the youthful character is retained in the 

 adult. The same condition exists in Salenia pattersoni (Plate 4, figs. 1,2). In Echinocyamus 

 (Love"n, 1874) there is a single external madreporic pore. In Habrocidaris scutata (text-fig. 

 206), as shown by Messrs. A. Agassi z and Clark (1908), no fine madreporic pores exist in genital 

 2, but, instead, there is a single pore in the middle of the plate, which appears to be a retention 

 in the adult of the single madreporic opening characteristic of the young. This is the only 

 case known in an adult regular echinoid. 



In Palaeozoic Echini a typical madreporite with numerous fine pores is known in a few 

 species. It exists in Lovenechinus lacazei, as observed by Bather (text-fig. 243); also, it has 

 been found in Lepidesthes formosa (Plate 68, fig. 5), L. colletti (Plate 71, fig. 1), Meekechinus 

 elegans (Plate 76, fig. 6), and Echinocy stiles pomum (Plate 18, fig. 6). In a number of Palaeo- 

 zoic genera the apical disc is not known well enough to enable one to state anything with assur- 

 ance. In the Palaeechinidae the apical disc is known in many species and yet the madreporite 

 has been seen in Lovenechinus only, as above noted, and reported in Melonechinus multiporus 

 by Keyes (1894). I have not seen a specimen of Melonechinus with madreporic pores. 



In post-Palaeozoic regular Echini the madreporic pores exist in genital 2, and are typically 

 limited to that plate. In examining large series of specimens, it is frequently found that the 

 madreporic pores extend beyond genital 2. When this occurs, in the great majority of such 

 variants, they extend to genital 3 and frequently ocular III as well (Centrechinus, text-fig. 188) ; 

 or they may extend to ocular III without invading genital 3 (text-fig. 146, p. 134). This 

 extension of madreporic pores has been seen in many species, but in Strongylocentrotus dro- 

 bachiensis, on account of the number examined, a greater range is known than in others studied. 

 Of this species in 33,000 specimens, 928 cases were noted in which madreporic pores extended 

 from genital 2 to 3 (Plate 5, fig. 13). In 128 cases madreporic pores occur in genital 2 and 

 ocular III (text-fig. 146, p. 134), and in 64 cases in genitals 2, 3 and ocular III, as in Centre- 

 chinus (text-fig. 188). In 80 cases, madreporic pores are in genitals 1, 2, 3, and in 60 cases in 

 genitals 2 and 1 (text-fig. 142, p. 134). Other variations in the extension of madreporic pores 

 beyond genital 2 are only rarely found. In 15 cases, they exist in genital 2 and ocular II; in 



