tS35,j arranged in Natural Farmlih* lOT 



at present is between the genera Malacota and Tubicinella, both 

 of which are subcyUndrical, although one evidently belongs to the 

 compressed, and the other to the depressed group of the class. 

 The families are susceptible of several methods of division, but 

 that used by Lister appears to be most natural : thus, in Anati-* 

 feridoi and Coronulidaij the base and support of the valves is 

 only a thin naked membrane ; while in the other three families, 

 it is more or less shelly; for although it is flexible in Pollicipe^ 

 didcdf its surface is always covered with shelly scales, and m 

 Pyrgomatida and Balanidcp, it is as completely shelly as the 

 valves of the body themselves, and the valves of the operculum 

 are articulated together, and most accurately fit the mouth of 

 the shells. 



/ The older naturalists were inclined to make too few species of 

 this class ; but the modern ones, in avoiding this fault, have gone 

 to the other extreme, by making too many. There is a very- 

 large collection in the Museum named by Dr. Leach, but 

 nearly all his names being new (and often two or three to the 

 same species), without the slightest reference to those of other 

 authors, they, therefore, cannot be adopted without great exa- 

 mination. The species are not very easy to determuie, as the 

 shelly plates of the Anatiferid^s ^nd Pollicipedida are exceedingly 

 apt to vary both in their form and surface, even in the individuals 

 of the same group. The shells of the Balanidcc dire also greatly al- 

 tered in their general form by the closeness of the neighbouring^ 

 specimens ; when close they become elongated (thus B.cylindra- 

 ceus)j and when scattered they are often depressed and spread 

 out at the base. The surface of the valves is also altered by the 

 structure of the substance to which they are attached thus. Ihave 

 a Barnacle on aPecten which is transversely ribbed, and another 

 on a piece of wood where the surface has all the lines of the 

 grain marked on it. The species of the Pyrgomatidm are often 

 overrun by the corals in which they hve and are thus destroyed, 

 and rendered almost useless as zoological specimens. 



' '/; aici'j .... Article III. i^JMiw^-^an^^^^'^'^o ^>'*^^" 



Explanatibn of ah Optical Deception in the Appearance of the 



Spokes of a Wheel seen through vertical Apertures. By P. M, 



, iRogetyMI). i?ES#r;> ,(J\^ith a Pia^feXo^^iaqu. siU io «-:»vi* v .9' • 



' ' A curious' o'pfical' deception take^'^JD^c^' \men a 'carriage 

 wheel, rolling along the ground, is viewed through the intervals 

 of a series of vertical bars, such as those of a palisade, or of a 

 Venetian window-blind. Under these circumstances the spokes 

 if^ tile ,%nie]^ 'i;^stead of appearing straigtit^j'^^^^^li^'^^^^ SHj^U?^ 



* From the Philoiophical Transactions for 1525, Part T, 



