506 LIMACINID^. 



of Pteropocl (^Spinalis Fleming-ii?} which had occurred in great 

 abundance at Nahant during the summer of 1863. His observa- 

 tions of the habits of these animals agree with those of Eang and 

 Souleyet. 



They come to the surface of the water an hour after dusk ; they do not 

 remain long, and after ten o'clock at night were rarely met with. He suc- 

 ceeded only once in finding a few isolated specimens during the heat of the 

 day ; while at full tide, soon after dark, they were very often found in abun- 

 dance. These animals are very easily kept in captivity, and their habits, 

 which can then be carefully watched, may explain in a very satisfactory man- 

 ner their sudden aj^pearance and disappearance. As was already previously 

 known these animals can creep about by means of their wing-like appendages. 

 When kept in captivity it was noticed that they but rarely left the bottom 

 during the day, merely rising a few inches, and then falling again to the 

 bottom of the jar. After dark, however, they could all be seen in great 

 activity, moving near the surface of the water as fast as their appendages 

 enabled them. During the day they often remain suspended for hours in 

 the water simply by spreading their wing-like appendages, and then suddenly 

 drop to the bottom on folding them. This habit of remaining at or near 

 the bottom, which they have in common with so many of our marine an- 

 imals, explains undoubtedly their sudden appearance and disappearance, as 

 they probably only come to the surface in search of food at certain hours. 

 When the animal is in motion, beating the water like a butterfly to propel 

 itself forwards or upwards, the shell is carried at right angles, hanging 

 somewhat obliquely to the direction of the movement. To counterbalance 

 this weight, an exceedingly long and powerful siphon extends on the op- 

 posite side of the animal, which is used as a kind of balance-wheel; the 

 shell, while the animal is in motion, assuming a totally different position 

 when it is not thus counterbalanced. Mr. Agassiz exhibited at the same 

 time drawings of the animal in different attitudes. 



The shell of this Fteropod resembles more *S'. Flemingii than the Spin- 

 alis GouUlii of Stimpson. This is the first time that a living Pteropod of 

 this family has been observed on this coast. 



The figures referred to are copied from the drawings made Ijy ]\Ir. 

 Alex. Agassiz and kindly loaned by him. 



Order GYMNOSOMATA. 



Body naked, without any shell. Head distinct. Wings two or 

 four, at the junction between the head and the body, with a central 

 intermediate lobe or rudimentary foot ? Gills exterior. 



