10 



KR. BONNEVIE 



[REP. OF THE "MICHAEL SARS" NORTH 



end of the tail; it may consist only of a pair of 

 cuticuiar-folds on the sides of the tail, the dorsal 

 ridge of the latter continuing to the end of the fin, 

 or it may form a horizontally extended heart- like leaf, 

 upon which the various ridges of the tail are not 

 continued. 

 9) The male organs: the shape of the penis in 

 mature animals, and especially the position of the 

 sucker relatively to the median line of the swimming 

 fin; it may be placed either at the middle of the 

 edge of the fin, or anterior or posterior to that point. 

 10) The radula is in the genus Pterotrachea of relatively 

 little importance as a specific character, the difference 

 between the radulae of distinct species being slight, 

 and the variations in one and the same species, on 

 the other hand, great. 



But, after all, a full description, or rather precise 

 drawings, of the different parts of the radula may be 

 of great importance in the recognition of a species. 

 The number and arrangement of spines on the median 

 plates, and on the free end of the intermediate plates, 

 may, together with other characters, be sufficient to 

 distinguish one species from another. 



Most of the characters mentioned above vary from 

 one species to another in the material investigated, 

 but a few of them are constant in whole 

 groups of species, and seem therefore to repre- 

 sent characters of subgeneric value. 



Such is the case with regard to the presence or 

 absence of a cutis-shield on the sides of the fore- 

 trunk, and of peribuccal teeth, and also with regard 

 to the position of the pedal ganglion. 



In these characters one species brought home by 

 the "Michael Sars" differs from all the others, a 

 distinction sharp and important enough to group the 

 species of Pterotrachea in my hands into two sub- 

 genera different from those proposed by Tesch. The 

 characters of these subgenera will be given below. 



Subgenus I. Heterodens includes species with a 

 dorsal cutis shield on the fore-trunk, with peri- 

 buccal teeth, and with the pedal ganglion in a 

 position considerably behind the anterior edge of the 

 swimming fin. 



Heterodens gegenbauri, Vayssiere. 

 (PI. IV, figs. 40—46). 



Eyes (fig. 41, text fig. A. 1 a) large, cylindrical, their 

 height about twice the diameter of the eye-lens. 



The nucleus (fig. 43, textfig. A. 1 b.) does not 

 rise above the level of the body, its length measuring 

 about three times its greatets width. 



The cutis is very clear and transparent, generally 

 without cuticular spots, but covered with cone-shaped 

 tubercles (concerning their distribution see below). No 

 cuticular spines on the forehead. The cutis-shield, cha- 

 racteristic of the subgenus, forms a thick gelatinous mantle 

 covering the back and the sides of the whole foretrunk. 

 Its greatest width is behind the eyes, and from this line 

 it gradually tapers towards the region of the swimming 

 fin. The anterior part of the trunk is, therefore, broadly 

 pear-shaped (fig. 40). 



Buccal teeth in two rows, 6—7 in each row. Peri- 

 buccal teeth irregularly distributed in a circle round the 

 entrance to the mouth (fig. 42 p. t.). 



The pedal ganglion (pg) is found considerably 

 behind the anterior edge of the swimming fin, at a distance 

 from this point of about Vs of the base of the latter (fig. 44 a). 



The osphradium (o) is placed in the middle dorsal 

 line, surrounded by a tuberculated cuticular wall (fig. 43). 



Gills, about 20 in number, forming a continuous 

 row posterior to the osphradium and nucleus. 



The dorsal muscle bands of the tail scarcely devel- 

 oped, the ventral ones also very thin. Two pairs of lateral 

 muscles, the most ventral of which is the broadest. Each 

 muscle band is covered with a row of cuticular tubercles, 

 the cutis forming longitudinal ridges along the dorsal 

 and ventral edge of the tail (figs. 43, 46). These ridges 

 are not continued upon the heart-shaped, leaf-like tail 

 fin (fig. 46 a-b). 



The swimming fin is semicircular, in the males 

 with a sucker placed anterior to its middle point, between 

 the first and second third of its border (figs. 40, 44 b). 



The radula (textfig. B. 1) consists of about 24 rows 

 of teeth. Median tooth with 5 gradually diminishing 

 spines on each side of the middle spine. Intermediate 

 plates with a small secondary spine at their free end. 

 Lateral plates nearly as long as the intermediate plates. 



Size of different individuals varying between 45 and 

 80 mm. in length, this variation greatly depending upon 

 the different degrees of contraction of tail and proboscis; 

 the variation in length of the trunk itself in full-grown 

 specimens is very slight, the average being about 40 mm. 



The characters mentioned above fully justify the 

 indentification of our species with the one described by 

 Vayssiere (1904) as P. gegenbauri. This author mentions 

 the cutis-shield, so characteristic of the species, - - a 

 character which is, however, not approved of by Tesch 

 (1906) as being of systematic value, representing in his 

 opinion only the hanging folds („Kehlsack") so generally 



