REVISION OF FRESHWATER SPONGES OF SPONGILLIDAE 19 



Gemmoscleres very robust and comparatively short amphioxea, 

 feebly curved or straight, entirely covered with stout, sharp, and 

 recurved spines; length range 65-86 n, width range 9-13 m- 



Gemmules very abundant in sponge, scattered throughout its body, 

 large and spherical, ranging 480-650 m in diameter; pneumatic layer 

 unusually strong, distinctly granular, very irregular in thickness; 

 outer gemmular membrane thin and produced into a number of radiat- 

 ing lobes of great extent; gemmoscleres comparatively sparse, em- 

 bedded in the thinner part of the pneumatic layer more or less 

 tangentially to irregularly, displaying a pronounced radial arrange- 

 ment in the thicker lobes of that layer; their tips do not project 

 beyond the outer gemmular membrane; foramen slightly elevated 

 extending to the surface of pneumatic layer, but not tubular. 



Distribution. — As yet known only from the vicinity of the type 

 locality, the cenotes of Yucatan, Mexico. 



Color in life. — Recorded by Old (1936a) as light gray to green; 

 the dry holotype of rather whitish color, resembling that of a dry 

 S. alba. 



Discussion. — This species was first recorded by Old (1936a), who 

 described it as S. lacustris Auctorum, and whose viev/s were per- 

 petuated by Rioja (1940a). Thus it happened that S. lacustris, a 

 typically cold-temperate species, became mistakenly added to the 

 Mexican spongillid fauna. S. cenota differs from S. lacustris in many 

 important criteria and cannot possibly be mistaken for it. On the 

 other hand it shares most features with S. alba, with which it forms a 

 separate group within the genus. Both S. alba and ^S. cenota possess 

 almost identical microscleres, as well as megascleres; in dry condition 

 both species are extremely similar, having a typically w^hite appear- 

 ance; both have the same close network of the skeleton, making it 

 much firmer than that of the *S'. lacustris group of species. S. cenota, 

 however, differs from all other known members of the genus, except 

 S. wagneri, by the peculiar, thick, and highly irregular pneumatic 

 layer of the gemmules, which forms distinct lobose projections armed 

 with radially embedded gemmoscleres; . and from aS*. alba by the 

 characteristic shape and structure of its gemmoscleres. 



Until speciation trends in freshwater sponges will be studied in 

 greater detail, possibly resulting in a trinomial nomenclature for 

 distant populations of apparently similar species, it is thought 

 advisable to elevate S. cenota to full specific rank, instead of relegating 

 it to a subspecies of S. alba. Its name is derived from the apparently 

 restricted locality, the cenotes of Yucatan. 



