458 ZOOLOGY. 



noue are known from the roeks intervening between tbe last and the 

 tertiary. Nevertheless, no reasonable naturalist will doubt that they 

 did exist in that intervening period, and few now would believe that 

 the devonian forms were not preceded by kindred types. The distri- 

 bution of the order is therefore a striking exemplification of the " imper- 

 fection of the geological record." (A. J. S., (3,) XX, 403-415; XXI, 

 125-128.) 



Shells of Lake Tanganyika. 



As a rule there is soraethiug in the appearance and texture or epi- 

 dermis of true fresh-water shells which enables the conchologist at once 

 to recognize them as such, even though they be quite different from any 

 forms he has i)reviously known; but in a Central African lake — Lake 

 Tanganyika — peculiar univalve shells have been discovered which a»e 

 remarkable for their unlikeness to any fresh- water forms before known, 

 and conversely for their resemblance to marine shells. The mimic forms 

 have been designated as follows : 



(1.) LiMNOTROCHUS (E. A. Smith, Ann. Mag. Nat. Hist. (5), v. 6, p. 425, 1880; P. Z. S , 

 London, 1881, 285). 



This is said to "have all the appearance of a Trochus when viewed 

 with the aperture from the eye. It is, however, more closely related to 

 the Littorinidoe, and exhibits the greatest affinity with the genus Eclii- 

 nella.''^ Two species have been discovered, one of which {L. Thomsoni) 

 has an especial resemblance to an Uchinella, while the other {L. Kirki) 

 reminds one, by form as well as sculpture, of a Solarium. 



(2.) Syrnolopsis (E. a. Smith, Ann. Mag. Nat. Hist. (5), v. 6, p. 426, 1880; P. Z. S. 

 L., 1881, 288). 



This "curious form has all the appearance of a marine genus, in fact, 

 closely resembling Obeliscus or Syrnola.''^ Mr. Smith believes "it may 

 temporarily be classed with the Eissoidse." 



(3.) TiPHOBiA (E. A. Smith, P. Z. S. L., 1880, 348 ; 1881, 293). 



This form is thought by Mr. Smith to be "perhaps the most remarka- 

 ble species of fresh-water mollusca yet discovered." It reminds one of 

 a "P^r?<?rt," but still more of a "jRo/jawcr," while it also may recall to 

 mind the Glotella armigera of the United States. 



(4.) Tanganyicia (Crosse, Jonrn. Concho!., (3,) t.21, p. 287, 1881). 



The species of this group somewhat resemble Natioids, but they also 

 approach the Ampuliariids. 



(5). Spekia (Crosse, Joum. Conchol. (3), t. 21, p. 302, etc., 1881). 



This "subgenus" of '■'■ Lacunopsis'''' has been proposed for the '■'■Litlio- 

 glyphus zonatus'''' of Woodward, a shell peculiar for its thickness, and 

 thus recalling certain Littoriuaj, but related to a fluviatile mollusk of 

 China. 



(6.) "Melania ( ?) Hoiei" (E. A. Smith, P. Z. S. L., 1881, p. $92). 



This shell is said by Mr. Smith to be " another instance of a species 



