BY T. HAEVEY JOHNSTON AND 0. W. TIEGS. 115 



Family III. MONOCOTYLIDAE Taschenberg, 1879. 



Small, slender or robust, medium-sized Gyrodactyloidea in which the glan- 

 dular head-organs are absent. The disc has developed into a suoker-like struc- 

 ture, a character which is already present in the Merizocotylinme. Major hooks 

 present, though apparently at times absent. Eyes present or absent. Testis 

 simple and compact or broken up into follicles. Ovary simple. Vagina present 

 (or absent?), generally paired. Intestine bifurcated, caeca present or absent. 



From the gills of Elasmobranchs. 



It is customary to include the Monocotylidae with the Tristomoidea, but the 

 affinities of the group are much more with the Gyrodactylidae, Merizocotyle 

 forming an intermediate link. 



The following genera belong to the family: — Monocotyle Tschbg., 1878; 

 Trionchus MaeCallum, 1916; Calicotyle Dies., 1850; Microbothrium Olsson, 1869; 

 Pseudocoiyle v. Ben. and Hesse, 1865; perhaps also Leptocotyle Montic, 1905. 



Key to sub-families of Monocotylidae. 



A. a. Disc about as broad, or a little broader than body Monocotylinae 



b. Disc much narrower than body B. 



B. a. Anterior end of worm narrow, posterior end very broad; disc divided by 



radii into several suckers and armed with hooks CaUcotylinae. 



b. Both ends attenuated, disc very small, devoid of radii, and of large 



hooks Pseudocotylinae. 



Subfamily I. MONOCOTYLINAE Gamb, 1896. 

 Slender Monocotylidae in which the disc is about as broad as body and pro- 

 vided with two or three major hooks. Testis compact. Vagina single (or 

 double ?) . 



In addition to Monocotyle, we axe including Trionchus in this subfamily. 



23. Monocotyle Taschenberg, _ 1878. 



Elongated worms with large posterior clasping disc, divided by eight radii 

 into as many suckers. Two major hooks and numerous minor hooks, the latter 

 scattered over the disc, especially at its margin. Mouth very large, situated at 

 anterior end. Vagina single (or double ?). Intestinal limbs end blindly. Eggs 

 oval and provided with a filament. From Elasmobranchs. 



Type s p e c i e s, M. myliobatis Tsch. 



Known species: — M. myliobatis Tsch., 1878 (Europe); M. ijimae Goto, 1894 

 (Japan); M. dasybatis MaeCallum, 1916 (U.S.A.). 



Monocotyle minima, nom. nov. 



(Syn. M. dasybatis minimus MaeCallum, 1916.) 



In 1916 MaeCallum described two species of Monocotyle from a stingray, 

 Dasybattis pastinacus, from Massachusetts, — viz., "M. dasybatis nov. sp." and 

 "M. dasybatis minimus nov. sp." Both are well figured and a glance is sufficient 

 to show that the two are quite distinct, not only in their dimensions but in 

 regard to the major hooks of the disc, the oral region, disposition of the ovary 

 and testis, etc. No suggestion was made by the author that the second species 

 was to be regarded as a variety or subspecies of the former, but the two were 

 evidently considered as quite distinct species. It seems to us that the second 

 name is a pure trinomial and accordingly invalid as a combination. In order to 



