74 U.S. NATIONAL MUSEUM BULLETIN 2 59 



Type locality.^ — A small spring on the south end of the National 

 Zoological Park, Washington, D.C. Rock Creek drains the Zoological 

 Park and is fed by a number of seeps and small springs along its course. 

 One of these ground-water outlets is the type locality, but it could not 

 be specifically located during the field work of May, 1965. 



Distribution and ecology.— /S. hayi is known only from its type 

 locality. Like S. t. potomacus, this species appears to be an inhabitant 

 of shallow ground waters associated with Precambrian rocks of the 

 Piedmont. The apparent rarity of S. hayi may be because of its 

 inability to successfully compete with S. t. potomacus, with which it 

 occurs sympatrically. On the other hand, this species might be more 

 closely associated with the hypogean environment, therefore dispersing 

 less readily into epigean habitats. Virtually nothing is known about 

 the biology of this species. Although brood plates were well developed 

 on a number of females in the type series, none were ovigerous. 



Remarks. — Hubricht and Mackin (1940) listed a small spring in 

 Montrose Park, Georgetown, Washington, D.C, as a locality for 

 S. hayi. I have examined this collection, however, and found it to 

 be conspecific with S. t. potomacus. Hubricht (1959), apparently 

 without critical comparison with type material, synonymized S. hayi 

 with S. tenuis. My studies, however, revealed that both of these 

 species are distinct. The earlier confusion of S. hayi with what is now 

 S. allegheniensis has already been discussed. 



In order to more clearly delineate this species, it has been necessary 

 to redraw some of the structures figured previously by Hubricht and 

 Mackin (1940). 



Stygonectes species (possible hybrid between S. tenuis and S. hayi?). 

 A collection of 7 males and 6 females made by W. H. Ball on Dec. 17, 

 1939, from a spring between Suitland and Forestville, Prince Georges 

 Co., Md. (USNM Ace. No. 158/311), contained specimens that 

 could not be referred to S. t. potomacus or to S. hayi. This material 

 is strongly indicative of hybridization between aS*. t. potomacus and 

 S. hayi which conceivably might have resulted from habitat dis- 

 turbance leading to a temporary breakdown m isolating mechanisms. 

 The largest specimen in this collection was a male, 9.75 mm long. 

 The largest female was 7.00 mm long. A detailed comparison of 

 the male with comparable sized males of S. t. potomacus and S. hayi 

 revealed the following: length of dactyls, posterior margins of bases 

 5-7, gill arrangement and spination of uropod 1 resembled S. hayi; 

 armature of posterior angles of propods of gnathopods 1 and 2, size 

 and form of penultimate segment of antenna 2, and possession of 

 tines on outer, apical spines of the telson resembled S. t. potomacus. 

 The following characters appeared to be intermediate in structure 

 between these two species: antenna 1 only about 5 percent longer 



