ANATOMY OF OPHTO'/HRTX VARIABILIS, ETC. 



115 



for there is but one, and it is concave at its lower part between 

 the lateral articulating surfaces. Indeed, the breadth of the 

 peg is very striking, as is also its amount of aboral projection. 

 The expansion on either side of the ridge is considerable ; and 

 in some bones there is a swelling almost knob-like at the outer 

 side of the usual socket for the knob of the next bone (PI. X. 

 figs. 16, 17, k). The adoral surface of a bone, corresponding to the 

 aboral surface just noticed, has a broad, bluntly triangular pro- 

 jecting umbo (u) beneath the slot-shaped surface of the apophysis 

 (PI. X. figs. 18, 19). Below the umbo is a considerable depression 

 for the median ridge of the opposed aboral surface, and the side 

 continuation and its enlargements. The knobs (k) are large and 

 project, and the cavity between them, and which merges below 

 into the inferior notch, and which is for the receptiou of the 

 broad peg, is rather large. The inferior notch is tall and wide. 

 The apophysis is bent forwards above and is convex below. The 

 upper muscle-fields of both surfaces are large, that of the aboral 

 predominating, and the lower fields are, as is usual, small and 

 oblique. 



V. Special and General Remarks upon the Muscles. 

 Some of the muscles of Opliioihrix variabilish.a,Ye already been 

 noticed in the description of the specimens, but it is necessary to 

 consider others and to refer slightly to all. It is evident that 

 the muscular development and distribution is not similar in all 

 Ophiuridse. Simroth has given the muscles of Ophiactis virens 

 his careful attention, and his descriptions are very valuable 

 ("Anat. und Schiz. der OpJiiactis virens, Sars," Zeitschr, f. wiss. 

 Zool. Bd. xxvii. p. 417, 1876). 



The muscles of the teeth (" interradiales adorales superiores "), 

 according to Simroth's type, arise from the upper part of the 

 adoral edge of the adjoining mouth-frames external to the jaw- 

 plate or torus. There is one muscle to a tooth, and it may have 

 a double insertion into the base of the tooth, there being two 

 horizontal foramina in the jaw-plate at the base of the tooth, or 

 a single one, the double foramen then being absent and repre- 

 sented by a slit. 



In Ophiotlirix variabilis, however, there are four and sometimes 

 five depressions, diminishing in size from above downwards, in the 

 substance of the adoral vertical edge of each mouth-frame (PI. X. 

 fig. 8) ; and consequently, when the two mouth-frames are com- 

 bined, there is a double series of cavities placed side by side. A 



