604 PROCEEDINGS OF THE NATIONAL MUSEUM vol. 115 



are recorded from Newfoundland and this assumption may be correct ; 

 however, the type specimen is not known to exist and it is not possible 

 to discern from the description just what species of Rhizoprionodon 

 Richardson might have had. Since the name has been used widely 

 for the North American species, it seems best to retain it here. 



The species occurs in marine and brackish waters at depths (over 

 depths?) as great as 153 fathoms. 



Relationships. — Rhizoprionodon terraenovae and R. porosus are 

 cognate species. The separation of these two species is based on 

 precaudal vertebral numbers (table 12) that are correlated with geo- 

 graphically non-overlapping distributions ofithe*species. Some minor 

 proportional differences are indicated in tables 11 and 13, but since 

 these are small, they seem scarcely of value in recognizing the species. 



The explanation for the existence and distribution of two cognate 

 species such as R. terraenovae and R. porosus is not clear. The Pleis- 

 tocene glacial rehct theory of Walters and Robins (1961) does not seem 

 to apply here as the two species apparently are common in both the 

 tropical and temperate portions of their distributions. Continued 

 separation is also a problem. The species are essentially shallow- 

 water forms, but some specimens of each species have been taken well 

 out to sea and in waters of great depth (see R. porosus). Even if 

 depth and expanses of water hindered mixing of the two species, it 

 would still be necessary to explain why they have not invaded each 

 other's territory from the Central American coast, where R. terrae- 

 novae is reported from Yucatan and R. porosus from Honduras. 



Rhizoprionodon terraenovae and R. porosus are related closely to R. 

 acutus, from which they differ primarily in having a shorter snout 

 length (fig. 1) and a lower dorsal-pectoral ratio (table 3). 



Material. — new Brunswick: Bay of Fundy, Grand Manan 

 Island, MCZ 178 (1: 400 mm.), new jersey: Holly Beach, ANSP 

 22058 (1: 642). Maryland: Baltimore, UZMK 358 (1: ca. 488). 

 VIRGINIA: Chesapeake Bay, Cape Charles, USNM 42491 (1: 530). 

 NORTH Carolina: MCZ 1320 (3: 296-362), NMV [no number] (6: 

 355-370); Fort Macon, ANSP 561 (1: ca. 285), MCZ 35209 (2: 313- 

 341); Beaufort, USNM 51892 (1: 378), USNM 51879 (1: 324), CAS 

 19777 (1: 344). south Carolina: Charleston, USNM 25181 (4: 

 437-545), MCZ 712 (1 : 312) ; 33°38' N. and 77°36' W., USNM 38511 

 (1: ca. 785). Georgia: 33°52' N. and 78°13' W., 5 fathoms, USNM 

 196179 (1: 631). Florida: USNM 39350 (1: ca. 915, skin); 29°47' 

 N. and 80°12' W., 145-153 fathoms, USNM 158479 (1: 335); Indian 

 River, USNM 7306 (4: ca. 333-ca. 375); 24°44' N. and 80°43' W., 30 

 fathoms, USNM 196801 (1: 758); Key West, USNM 125883 (1: 550); 

 Tortugas, USNM 61126 (1: 228, embryo); west coast at 26°10' N., 

 26 fathoms, USNM 196168 (1: 621); west coast at 26°44' N., 29.5 



