46 Mr. W. Clark on the Skeneadae. 



long nor slender flattish tentacular filaments issuing from tuber- 

 cles of the same elegant structure as the capitular ones ; these 

 are not vibrated with the usual activity of the tribe, but the 

 curved auricles of the foot may be said to be " Isete vibrantes." 

 The genitale springs under the right tentaculum ; it is flat pointed 

 and lies horizontally, nearly extending to outside the aperture, 

 not reflected in the branchial vault. The canal of depuration is 

 visible at the right side just above the first vibraculum ; it is a 

 short pendent shoot or cylinder. This animal inhabits the coral- 

 line zone in fifteen fathoms water, five miles off Budleigh Sal- 

 terton ; it is active, marches with quickness, not at all shy, and 

 gave me good opportunities of observing its points. 



It thus appears that the principal difibrences between this 

 species and its congeners are the mere specialties of the want of 

 distinct eye-pedicles, and the long linear curved auricles of the 

 foot. Axis ^j, diameter -^^^ uncise. 



This very important discovery of a desideratum that has 

 hitherto escaped detection proves that the animal is nearly a 

 strict Trochus, which does not in the specialties show a greater 

 departure from the trochidian type than is often seen amongst 

 the most classic species. This fact determines the fate of the ge- 

 nus Skenea : its provisional members, the S. Cutleriana and S. 

 nitens of Philippi, called by some authors " Trochus piisillus," are 

 in all probability Trochi ; but I will not venture to say as much 

 of S. nitidissima. The S. lavis is scarcely a variety of our present 

 species. The S. costulata is apocryphal. 



Professor Forbes, when he deposited these species provisionally 

 in Skenea, with infinite sagacity predicted that they would pro- 

 bably prove Trochi ; he is right, at all events, as to the one he 

 considered would when discovered determine the position of the 

 others. I did not concur in this opinion, as I thought the en- 

 tirety of the aperture and its want of angularity did not harmo- 

 nize with the typical Trochidse ; my conjectures have not been 

 confirmed ; but I feel pleasure in having the good fortune to dis- 

 cover my own error, and verify the acuter perceptions of this 

 profound naturalist. 



Can the genus Skenea be maintained even for the so-called 

 " planorbis" ? which I have for the second time just examined; it 

 appears to be absolutely a discoid Rissoa, allowing the necessaiy 

 margin for specialties of the shape of the foot, operculigerous lobe, 

 tentacula and opercula : these organs greatly vary in the Rissoa, 

 and often differ more with each other, and the type, than even the 

 discoidal " planorbis." Ought there not to be two sections in 

 Rissoa, — one for the elongated Cerithium reticulatum, which re- 

 peated examinations tell me does not exhibit a difference from it 

 in any material point, and might as respects the animal be the 



