720, REPORT OF COMMISSIONED -\i AND FISHERIES. [2] 



r 



iiave been < -st useful to me in the work of identification, are rbose 

 which give ;u;ry details of structure and are accompanied with ill .,< ; ;i- 

 tions. Whether the name given y the describer holds or not is a 

 ter of secondary importance. 



I have restored Van Beneden's genus AcantJiobothrium, which hrd 

 been combined with the genus Cattiobothrium by Diesing. This neces- 

 sitates au emendation of the definition of the latter genus. 1 have 

 ited from the genus Echeneibothrmm those species with echenci- 



. which are destitute of a myzorhyuchus, and placed t.b 

 in the new gen as Rliiiiebo. Throe genera, in \vhich thebji* 



are united into a globe or disc were discovered, whose systematic ! 

 tions are open to some debate. These have been named Lecanicephalum, 

 Tyl' '>rin, and Discocephalum^ respectively. The fa:nily name 6V 



bothriidiv is suggested for-these forms, although I have thought bes 1 

 put them provisionally with the Tetrabothriidcc. The species which I 

 have described under the name Paratcenia medusia has caused mo much 

 perplexity, to determine its relationship. 



The specimens which are described in this paper were collected 

 the most part, during the months of July and August 1S86-'S7, at 

 "Wood's Holl, Massachusetts. During tin summer of 1S87 I made most 

 careful and painstaking search for small forms, and was eminently s 

 cessful in my examination of the sting ray (Trygon centrura) and 

 dusky thark (Carcharias obscurus). During these researches a variety 

 of encysted forms were obtained. These were most abundant in the 

 Telcostei. Several species of Trematods, Nematods, and Acanthocephaia 

 have been found. Descriptions of these will appear in due time. L have 

 learned by experience that brief descriptions of these soft-bodied and 

 variable forms are of but little use in identification, and have therefore 

 endeavored to give such descriptions 'as will enable fntnre investigators 

 to identify the species accurately. It has been found that measi' 

 ments, even of parts that are liable to great alteration on accouir: 

 contraction, are invaluable as a means of identification. Measure 

 of hard parts, such as hooks, spines, and, to a certain extent, o 

 of course of the highest importance. Too much weight, "u-. .vever, 

 not be attached to absolute values where the differences are 

 Different methods of obtaining measurements, inaccuracies in com* .ifa- 

 tion, and individual errors must be allowed for. On the other hand, 

 much weight must usually be given to relative dimensions, since in that 

 , several of the above-named sources of error are eliminated. 



A-; ! tr as it was^ possible to do so the specimens were studied v, 

 they wore alive. Sketches of living forms were made by my wife 

 I was engaged in collecting, assorting, measuring, and recording 



LI ns on the specimens. I was thus c-uibied to collect much in- 

 i in the short time at my disposal than would have been possil 

 without this assistance. 



It may not he ann'ss to give hore. for flic ben >6t '-ctors, the 



