720 REPORT OF COMMISSIONER OF FISH AND FISHERIES, [2] 



have been most useful to me in the work of identification, are those 

 which give many details of structure and are accompanied with illustra- 

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

 matter of secondary importance. 



I have restored Van Beneden's genus Acanthobothrium, which had 

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

 sitates an emendation of the definition of the latter genus. I have 

 separated from the genus Echeneibothrium those species with echenei- 

 form bothria, which are destitute of a myzorhynchus, and placed them 

 in the new genus Rhinebothrium. Three genera, in which the bothria 

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

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

 Tylocephalum, and Discocephalum, respectively. The family name Gamo- 

 bothriidw is suggested for these forms, although I have thought best to 

 put them provisionally with the Tetrabothriidce. The species which I 

 have described under the name Paratcenia medusia has caused me much 

 perplexity, to determine its relationship. 



The specimens which are described in this paper were collected, for 

 the most part, during the months of July and August 1886->87, at 

 Wood's Holl, Massachusetts. During the summer of 1887 I made most 

 careful and painstaking search for small forms, and was eminently suc- 

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

 dusky shark (Garcharias obscurus). During these researches a variety 

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

 Teleostei. Several species of Trematods, Nematods, and Acanthocephala 

 have been found. Descriptions of these will appear in due time. I 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 future investigators 

 to identify the species accurately. It has been found that measure- 

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

 contraction, are invaluable as a means of identification. Measurements 

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

 of course of the highest importance. Too much weight, however, should 

 not be attached to absolute values where the differences are slight. 

 Different methods of obtaining measurements, inaccuracies in computa- 

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

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

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



As far as it was possible to do so the specimens were studied while 

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

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

 servations on the specimens. I was thus enabled to collect much more 

 data in the short time at my disposal than would have been possible 

 without this assistance. 



It may not be amiss to give here, for the benefit of collectors, the plan 



