OF THE ACALEPHiE OF NORTH AMERICA. 289 



The new genus of naked-eyed Medusae to which I have alluded, and which resem- 

 bles Hippocrene so closely, differs from it in having a more movable and bottle-shaped 

 central digestive cavity, which may be more or less protruded from the main cavity of the 

 body, and is not so persistent in its form as that of Hippocrene. The tentacles are ar- 

 ranged, as in Hippocrene, in four bunches, with eye-specks at their base ; but there are 

 two of these eye-specks supported upon two distinct stalks, rising above 

 the others and above the tentacles, similar in appearance to the protruding 

 eyes of a snail. Unfortunately, I was unable to have correct drawings of 

 this animal made at the time when it was first observed. I subjoin a 

 mere wood-cut, from a drawing made hastily at the time, by my son. 

 Master Alex. Agassiz. The specimens were caught in the harbour of 

 Nantucket, in June, 1849. For the genus I propose the name of Nemopsis, and shall 

 call the species N. Bachei. 



TIAROPSIS. 



In its general outlines, the species upon which this genus rests is very plain. It has 

 a hemispherical body of perfectly regular form, but of a very elegant appearance in its 

 beautiful simplicity. Imagine a convex disk of transparent jelly, with a slight bluish milky 

 tint, from less than half an inch to nearly an inch in diameter, somewhat like a deep watch- 

 glass, with its margin fringed all round, and with the centre somewhat thicker, having a 

 short proboscis in the middle, spreading into fringed lobes, from the upper side of which 

 arise four tubes, diverging at right angles to meet the margin, with an elliptic or fusiform 

 flattened swelling about the middle of their course, and eight deep black specks along 

 the margin between the marginal tentacles, at the base of each of which there is a small 

 black speck, and you will have as correct an image of the general appearance of this pretty 

 little Medusa as a first impression may leave. When it is in motion, however, its form 

 varies to a considerable extent, by the contraction of the muscles. When relaxed (Plate 

 VI, Fig. 1), it is a very flat, saucer-like convex disk. When contracted (Plate VI. Fig. 2, 

 3, and 4), it assumes a more hemispherical shape, and when highly contracted, approaches 

 nearer and nearer to a globular form. But in its state of high contraction (Plate VI. Fig. 6), 

 the peripheric part of the disk is more or less detached from the central part by the stronger 

 contraction of a muscular ring, forming a circle on the outer surface of the body aix)ve the 

 tentacles. These forms pass gradually from one into the other: the margin may even be 

 spread, and the centre so contracted as to give it a more globular form, when the outline of 

 the animal assumes rather the shape of a church-bell. The circular outline of the whole 

 body nviy also be somewhat modified, and assume a subquadrate appearance (Plate VI. 



