3334 THE MOLLUSKS OR SHELLFISH 



flat shell is concealed beneath the mantle, over the back. P. (oscanias) membranaceus 

 and P. plumula have both been found on various parts of the British coast. Both 

 range as far as the Mediterranean. The spawn of P. plumula is deposited in rib- 

 bon-like and spirally-arranged masses, resembling those of Doris. 



The animal of the genus Umbrella, which typifies another family, is very large, 

 and carries upon its back a flat, circular, external shell, recalling the form of an 

 umbrella of the Chinese pattern. The gill, as in Pleurobranchus , is on the right side, 

 beneath the mantle, and protected by the shell. This unsightly creature is remark- 

 able for the enormous development of the foot, which extends everywhere beyond 

 the margin of the shell, and practically incloses the head in front. One species 

 occurs in the Mediterranean, and three or four others are met with in the Indian and 

 Pacific Oceans. The shell of U. indica is sometimes fully five inches in diameter. 



The families Siphonariidce , Gadiniidce, and Amphibolidcz, already mentioned, 

 constituted, until recently, a suborder of Pulmonata to which Gray gave the name 

 Thalassophila. According to Haller, however, Siphonaria and Gadinia should be 

 regarded as modified Opisthobranchiates, and placed next to the Umbrellidce; but 

 their systematic position cannot be considered definitely settled. The Siphonariidce 

 look like ordinary limpets, and attach themselves to rocks in the same way at 

 about high-water mark. The shells are distinguishable by a groove on the right 

 side of the inner surface, interrupting the muscle by which the animal is attached. 

 Siphonaria much resembles Auricula as regards the pulmonary cavity, which is 

 adapted for breathing air or water, there being both a lung and a modified gill. It 

 is, however, protected or closed by a valve-like lobe of the mantle. The horny jaw 

 has a smooth and curved cutting edge. The radula consists of a single central or 

 rachidian tooth and numerous laterals, very similar in type to that of certain groups 

 of Pulmonata. About fifty species are known. The genus has an extensive range 

 from Vancouver island in the north to Cape Horn in the south. One species occurs 

 on the coast of Portugal, but the majority inhabit eastern seas. In Gadinia the 

 breathing chamber is without any sign of branchiae. It is a limpet-like animal with 

 a circular foot by which it attaches itself to rocks or stones, in the same manner as 

 limpets. It is probably a poor traveler. The pulmonary orifice is placed on the 

 right side near the head, and is closed by a small valve. The head is large, with- 

 out tentacles, with the sessile eyes placed at the sides. Mr. Ball, who watched a 

 colony of G. reticulata, noticed that "as long as the rock on which they were re- 

 mained damp, they continued with the margin of the shell firmty applied to it. As 

 soon as the bowlder became dry, under the hot sun, I perceived a simultaneous mo- 

 tion in the colony. Each shell was raised above the surface of the stone, the head 

 and foot were protruded, and the orifice of the pulmonary cavity was expanded. 

 They were evidently enjoying the warm air." 



SUBORDER Pteropoda 



Until within the last few years, the mollusks included in this suborder were 

 considered to constitute a separate class. Some systematists located the group near 

 the Cephalopoda, others, believing them to be of a more degraded type than the 



