ON THE APPENDICES C.KNITAI.KS (CLASPERS) IN THE SELACHIANS. 49 



The number of the terminal pieces is three; but to be able to see these pieces, and cor- 

 rectly to understand their relations to the marginal cartilages, it is necessary to remove a large 

 ventral covering piece, homologous with that mentioned in Rliina, and marked v, because the whole 

 ventral side of the terminal part is hidden as by the half of a thimble (the contour of which may 

 often be distinguished through the skin). The foremost edge of this covering piece is incised in an irre- 

 gularly heart-shaped manner, the one — lateral — corner being produced much farther than the other; 

 it is strongly rounded from side to side, and the surface is dotted with small holes. The edges of 

 this piece have on each side a peculiar bend, and immediately behind this the covering piece 

 is firmly joined to the specially thickened hinder edges of the two terminal pieces Td and Tv; 

 bv the said bends and the ventral concavities of the covered terminal pieces, the peculiar side- 

 slits and the walls of the before mentioned pockets* are formed. The M. dilatator is attached 

 to the fore edge of the covering piece, and by the firm connection between the covering piece 

 and the outermost point of the terminal pieces the action of the muscle is transmitted to 

 that point. 



The dorsal terminal piece, Td, is quite short, somewhat convex towards the appendix-slit; the 

 ventral side on the contrary is deeply concave; together with the corresponding marginal cartilage 

 which, as we have seen, is also hollow, it forms a complete trough, in which the medial «pocket is 

 situated. The ventral terminal piece, Tv, has the double length, is likewise rounded on the dorsal side, 

 hollow on the ventral one, and forms with the covering piece the lateral -pocket. Finally a piece 

 T~ is added to the hindmost end of the ventral marginal cartilage, and to the proximal part of Tv; 

 it is shaped as a slightly bent, round thorn, almost hidden inside of the lateral edge of the cover- 

 ing piece 1 ). 



The muscular system. The M. extensor is divided into two parts reminding of the state 

 in R/iiua. The foremost one originates from the foremost half of the basale, runs over the knee, and 

 is attached immediately below this to the M. dilatator, and partly to the distal end of ft and of the 

 appendix-stem; the hindmost one originates from the other, hindmost, half of the basale and from the 

 following pieces (also from ; S), passes with its distal part under (i.e. ventrally of) the foregoing one, 

 and is continued rather directlv in the medial part of the M. dilatator. 



M. compressor. The bag-shaped part of this muscle, situated on the ventral side, is very small; 

 properlv speaking it is confined to a ventrally rounded swelling between the two hindmost rays and 

 the parts of the stem-skeleton, 5 Z and 6 2 ; accordingly there is no part projecting across the ray muscles, 

 the foremost contour of the bag being bordered by the skeleton (the pieces b l and b,), the outermost 

 one by the hindmost ray muscles. The ventral fibres of this little bag run obliquely backward towards 

 the hindmost ray, almost in continuation of the hindmost fibres of the M. adductor. The part form- 

 ing the outer lip-muscle, is rather powerful and long, originates forward from the piece /? (foremost 



■) Petri, as already mentioned, is quite wrong as to the terminal part; he takes the dorsal terminal piece Td to 

 be the terminal joint of the appendix-stem (I.e. p. 308, fig. 4 F, & lv ); the ventral terminal piece and the thorn 7~ 3 become 

 zwei kleiue verkalkte Spaugen», borne by the hindmost part of the ventral marginal cartilage to which they are added 

 ■ radienartig 1 fig. 4 E and D, sp" [= Tv], sp' [= 7j]). Only the piece v (fig. 4 E, F, D sch) has been tolerably correctly 

 described and interpreted by Petri. Thus he has had no idea of the homologies of the terminal pieces with those in Acan- 

 Ihias and Scy Ilium, not to speak of Raja. 



The Ingolf-Expedition. II, 2. J 



