so-called Eyes o/" Tridacna. 437 



preservation have supplemented each other very advanta- 

 geously for the investigation. 



Now, therefore, we maj pass to the consideration of the 

 " eye-tentacles." At the very first glance at the margin of 

 the mantle of a Tridacna we observe a series of irregularly 

 formed but generally obtusely conical tubercles or warts, 

 which, at variable distances from the margin, upon the inner 

 or branchial surface of the mantle, form a row nearly parallel 

 with the margin of the mantle, and in my largest example, 

 in which the mantle-margin is about 18 centim. long, I count 

 on each side about fifty of these structures. Their distance 

 from each other and from the margin of the mantle is no less 

 irregular than their size and form. While the distance from 

 the margin usually varies between 2 and 5 millim., we find 

 individual warts much further inwards, even as far as 15 

 millim. The distance of the warts from each other is equally 

 variable. While we sometimes find groups of six or eight 

 together in a close series, a more irregular arrangement in 

 small groups of two or three placed at variable distances 

 apart is by far the most frequent condition. 



The form of the larger elevations is generally that of a 

 low hill, which, however, appears seated upon the surface of 

 the mantle not straight, but obliquely, in such a manner that 

 the apex looks towards the margin of the mantle. In the 

 largest structures of this kind the long diameter (by which I 

 mean that perpendicular to the mantle-margin) is usually 

 somewhat greater than the transverse diameter (parallel to the 

 margin), which it may exceed by about one third; in middle- 

 sized tubercles the two diameters are nearly equal, and in 

 small ones the proportion may be in favour of the transverse 

 diameter. Tu the largest v\arts observed by me the diameters 

 in question attained the lengths of 3 and 2 millim. As 

 regards the form of all the warts, of whatever size, it is 

 characteristic that their dorsal surface melts very gently and 

 gradually into that of the inner surface of the mantle, while 

 the ventral surface (that turned towards the mantle-margin) 

 descends abruptly, and, indeed, below the level of the surface 

 of the mantle, each wart being surrounded on its ventral side 

 by a semicircular furrow, which stands in the same relation 

 to it as the fosse of a fortress to the bastion. This fosse is 

 very seldom faintly marked or quite efiaced.' 



Between the series of large warts and the margin of the 

 mantle there is a series of smaller structures of a peculiar 

 kind, which are only just visible with the naked eye. Some- 

 times, but not frequently, the structures now to be described 

 occur also between the larger warts or even beyond them j 

 Ann. & Mag. N. Hist. Ser. G. Vol. i. 30 



