t^OME MOI.LUSCAN INHABITANTS OF THE NATAL 



LAGOONS. 



By 



F. G. Cawston, IM.D. Cantab. 



Read July 11, 1922. 



Visitors to the mouth of the Umgeni Eiver and excursionists 

 on the Durban Bay, or at Isipingo frequently remark on the 

 large number of shells attached to the trunks of ti-ees or lying 

 free on the mud when the water has subsided. 



Ccrithidca dccollata is a dark brown long spiral shell that 

 clusters in great numbers on the mangroves in the estuarial 

 swamps all along the Natal coast. Mature specimens are com- 

 monly an inch-and-a-quarter in length. At Isipingo I have found 

 it associated with Littorina scabra L, and Asse7nania hifasciata 

 Nevill. In the Durban Bay ,1 have found it with Littorina scabra, 

 Alcctrion hraussianus Dunker, Natica marochiensis Gmelin, and 

 Vole ma paradisaica lieeve. In the Illovo and Umgeni lagoons, 

 I found it associated with TJteodoxus nataJcnsis Eeeve. 



No cercariae could be detected in the rather brackish water in 

 which these examples were found. I found a shell of the genus 

 Cerithidea lying on the mud of the Kamdene lagoon. It was 

 not quite mature and only slightly decollated. Mr. Henry C. 

 Burnup stated that the scidpture is very different from that of 

 C. decollata and writes: " I am fully convinced it is the long lost 

 Cerithidea incequiscalpta Kob. The spiral grooves are wider apart 

 and tersect ths transverse ribs in your shell, while in decollata 

 they are finer, more numerous and are confined to the inter- 

 costal spaces. An occasional rib in your shell is much finer than 

 its fellows. This feature might suggest the specific name, 

 incBquisculpta. 1 believe it has never been collected except by 

 the discoverer, Freytag, some time prior to 1893, the date of the 

 publication of the desciiption, and he only got one worn specimen 

 at Durban. The locality has been doubted through the shell 

 never again having been found." 



Another common operculated shell from the Natal lagoons is 

 Theodoxus natalcnsis Reeve, which I have found in large numbers 

 in the Tongaat and Umkomaas lagoons. One example from 

 Tongaat was not quite an inch high, one from Umkomaas ^ in. 

 I have collected examples from the Umbogintwini, Illovo and 

 Umgeni lagoons. In regard to one I sent him from the last 

 locality, Mr. Burnup says: "The Theodoxus natalensis is the 

 largest I have, save one from Umkomaas. It is also the highest 



