2266 Bulletin 47, United States National Museum. 



the top of the head and neck, and is composed of a double series of 

 transverse, movable, cartilaginous plates, serrated on their posterior or 

 free edges. By means of this disk these fishes attach themselves to other 

 fishes or to floating objects, and are carried for great distances in the 

 sea. Opercles unarmed. Pectoral fins placed high ; ventral fins present, 

 thoracic and close together, I, 5; dorsal and anal fins long, without 

 spines, opposite each other; caudal fin emargiuate or rounded. Branchi- 

 ostegals 7. Gills 4, a slit behind the fourth; gill rakers short; gill mem- 

 branes not united, free from the isthmus. Pseudobranchia? obsolete. 

 Several pyloric appendages. No air bladder. No finlets. No caudal 

 keel. Vertebra? more than 10 -f- 14. Genera 4 ; species about 10, found in 

 all seas, all having a very wide range. The species of this group are 

 apparently descended from a fossil genus, Opisthomyzon,* Cope (glaron- 

 ensis), characterized by the small posterior disk and slender body. 



The following description of this family is given by Dr. Gill: Body 

 elongated, subcylindrical, diminishing backward gradually from the head 

 and into the slender caudal peduncle. Anus subcentral. Scales cycloid, 

 very small, and not, or scarcely, imbricated. Lateral line nearly straight 

 and very faint. Head above oblong and with a flattened straight upper 

 surface, furnished with an adhesive oblong or elongated, laminated disk. 

 The eyes are rather small, submedian, and overhung by the disk. Subor- 

 bital bones forming a slender infraorbital chain; the first or preorbital 

 triangular and thick. Opercular apparatus normally developed and 

 unarmed. Nostrils double, close together. Mouth terminal or, rather 

 superior, the lower jaw projecting, but with the cleft nearly horizontal 

 and not extending laterally to the eyes. Teeth present on the jaws and 

 palate. Branchial apertures ample and fissured forward. Branchiostegal 

 rays 7 (or 8) on each side. The adhesive disk on the upper surface of the 

 head is a modified first dorsal fin, and from the snout generally extends 

 more or less posteriorly on the nape and back; it is oblong or elongated 

 and of an oval or elliptical form, divided into equal halves by a longi- 

 tudinal septum, and with more or less numerous transverse lamina? in 

 each division, the laminae being slightly erectile and depressible. Dorsal 

 fin oblong or elongated on the posterior half of the body (including head), 

 ending some distance from the caudal. Anal fin opposite and similar to 

 the dorsal. Caudal fin rather small, variable in outline, but never deeply 

 forked. Pectoral fins moderate, inserted high on the sides. Ventral fins 

 thoracic, each with a spine and 5 branched rays. The vertebral column 

 has vertebra? in slightly increased numbers, the abdominal vertebra? being 

 about 12 to 14 and the caudal 15 or 16. The stomach is ca?cal and the pyloric 

 ca?ca are present in moderate numbers. The air bladder is obsolete. 



* "A careful comparison of the proportions of all the parts of the skeleton of the fossil 

 Echeneis with those of the living forms, such as Echeneis naucrates or Echeneis reniora. 

 shows that the fossil differs nearly equally from both, and that it was a more normally 

 shaped fish than either of these forms. The head was narrower and less flattened, the 

 preoperculum wider, but its two jaws had nearly the same length. The ribs, as also the 

 neural and haemal spines, were longer, the tail more forked, and the soft dorsal fin much 

 longer. In fact, it was a more compressed type, probably a far better swimmer than its 

 living congeners, as might be expected, if the smallness of the adhesive disk is taken into 

 account." (Storms.) This form (Echeneis glaronensis, Wellstein) is made the type of the 

 genus Opisthomyzon, Cope, the name referring to the posterior portion of the small disk. 

 The vertebrse in Opisthomyzon are 10 + 13 = 23. 



