298 CONTRIBUTIONS TO THE NATURAL HISTORY 



of Stricture between the upper disk and its margin. The circumstance that such con- 

 traction is observed in almost every dead specimen, while we hardly ever see the circular 

 outlines of the lower margin deeply lobed or contracted into quadrangular outlines, 

 plainly shows that in this type the circular muscles are more powerful than the radiating 

 bundles of fibres. At the utmost, we notice slight undulations in the lower outline of 

 the tentacular margin, between the radiating tubes, reminding us of the greater strength 

 of the inner radiating muscles ; the outer seem so delicate as to come into play only in 

 connection with the inner, and perhaps to stretch the margins in the state of relaxation 

 of the circular bundles and the lower radiating ones. At all events, the greater looseness 

 of the outlines, the thinness of the disk, especially towards its margin, and the greater 

 range of changes in the regular forms, agree with the particular modes of moving and 

 feeding characteristic of this genus. 



It were now very interesting to ascertain how far, among the numerous types allied 

 to those which occur on the British shores, there is any resemblance in the arrangement 

 or the greater development of their eye-specks, and the peculiar structure which charac- 

 terizes the genus Tiaropsis. From Professor Forbes's descriptions, there seem to be no 

 differences in the size of the eye-specks in any of his species of Thaumantias, to which 

 genus I would have referred my Tiaropsis, were it not for the existence of two kinds of 

 eye-specks around the margin, and the peculiar structure of the larger ones. I have given 

 the name of Tiaropsis to this genus on account of the very peculiar structure of the eye- 

 specks, of which no other example is yet known among Medusae. For the species I would 

 propose the name of diademata, as expressing more particularly the double wreath of the 

 crown-like eye-bulb. The species is extremely common along the wharves of Boston 

 harbour, and indeed throughout the bay, where it is constantly found near the surface, 

 amon"' Sarsiae and young Ephyrse, early in spring and until about midsummer, when 

 they disappear, soon after having laid their eggs. 



In an ideal vertical section of Tiaropsis (Plate VIII. Fig. 11), corresponding in its po- 

 sition to that of Hippocrene, Fig. 3, and to that of Sarsia, Fig. 5, the relative position and 

 natural connections of parts are illustrated in such a manner as readily to show the generic 

 distinction between these types, and the homology of their different systems of organs. 

 Without entering into any further details upon this subject, I will only mention that the 

 letters in all the figures correspond precisely to each other, and may therefore, with ease, 

 be referred from one genus to the other. 



