REPORT ON THE PTEROPODA. 35 



Posterior Gill a simple hexagonal crest, from the four ventral and dorsal angles of 

 which there arise four short crests, without foldings or fringes, directed forwards. (In 

 one specimen I only found three radiating crests, two dorsal and one ventral ; perhaps 

 the two ventral crests were fused together, or the left one was absent.) This gill was 

 unknown to Troschel, who founded his genus Cliopsis on young specimens not more than 

 8*5 mm. long, 1 in which the place of the gill was still hidden by the posterior ciliated 

 ring ; 2 but it was figured by Costa in 1873. 3 



Proboscis very long; in the figured specimen (PL III. fig. l), it is not yet quite 

 everted, since the radula, jaw, and hook-sacs do not appear at its extremity. 



Radula. — The formula is 6 : 1 : 6; the formula 4:1:4 given by Troschel 4 is that of 

 young specimens ; and even for them it is probably inexact, because Macdonald has 

 found that larvae measuring but 3 mm. (PL II. fig. 9) already possessed five lateral 

 teeth on each side, the most distal being exceedingly small. 



Hook-sacs. — Each containing about sixty hooks. 



Colour. — The animal is colourless, rather transparent, with numerous small brown 

 spots (in the specimens preserved in alcohol) visible to the naked eye, and due to 

 cutaneous fatty glands. 



Length. — Up to 24 mm. 



Habitat. — Central parts of the Mediterranean Sea ; Naples, Messina, and south of 

 Sicily, 20 miles east of Malta, lat. 36° 1' N., long. 15° 5' E., where Macdonald 

 found larvae. 



Observations. — I have already said that the genus Trichocyclus is not a good one, 

 and that the forms described under this name are larvae of different genera of Gymno- 

 somata. All the Gymnosomata indeed, in the second part of their embryonic develop- 

 ment, exist as naked larva?, with three ciliated rings. 



I may state that Trichocyclus mediterraneus, Costa, 5 is the larva of Clionopsis 

 krohni; a fact that I was able to ascertain by original drawings, much more perfect 

 than Costa's figure, which Dr. J. D. Macdonald kindly sent to me. One of these 

 drawings (PL II. fig. 9) represents a larva, 3 mm. long, with the two posterior ciliated 

 rings still attached, and one may say that its foot is similarly formed to that of the 

 adult Clionopsis krohni, and that there is no longer a posterior lobe. The axial 

 visceral nucleus is another proof which shows that this larva belongs to the genus 

 Clionopsis, as do also the radula (of which the formula is then 5:1:5), and the form of 



1 Archivf. Naturgesch., Jahrg. xx. pi. x. fig. 1. 



2 Von Iheriug therefore thought that Clionopsis was the " Jungstadium" of Clione, instead of a good genus 

 (Vergleichende Auatomie des Nervensystems und Phylogenie der Mollusken, pp. 245, 24(i). 



3 Pteropodi della Fauna di Napoli, pi. v. fig. 8. 



4 Beitrage zur Kenntniss der Pteropoden, Archiv f. Naturgesch., Jahrg. xx. p. 231. 

 6 Anniuvrio del Mus. Zool. d. R. Univ. d. Napoli, t. iii. p. 46, pi. i. fig. 3. 



