266 



ON TIVELA AND GRATELOTJPIA. 



By A. J. Jukes-Ueowne, F.R.S., F.G.S. 



Read 8th November, 1912. 



As tlie dentition of these shells presents exceptional fcatuivs and as 

 1 find that erroneous views regarding the structure of tlieir hinges 

 are largely current, a note on the subject, with the special object of 

 fixing the taxonomial rank of the fossils known as Grateloupia and 

 Cytheriopsis, seems to be required. 



With respect to Tivela, no one seems to have fully and correctly 

 described the positions of the cardinal teeth, nor tlieir complete 

 separation from the accessory dentiform ridges or rugosities. Most 

 writers, indeed, such as S. P. Woodward, Deshayes, Adams, and 

 Dall, have regarded all these dentiform ridges as cardinal teeth, and 

 have stated that the teeth vary in number from 3 to 7 in each valve. 

 Others, like Fischer and Cossmann, seem to have mistaken the first 

 supplementary tooth or ridge for the normal posterior cardinal, and 

 to have overlooked the small and slender anterior cardinal of the 

 right valve. 



Having examined the dentition of many species of Tivela, both in 

 young and adult specimens, and having compared it with that of 

 Meretrix and Sunetta, which are the most nearly allied genera, I can 

 state with confidence that the three normal cardinal teeth of those 

 genera are present in all species of Tiveia, although their position 

 varies, because they are more or less displaced by the growth of the 

 supplementary teeth or ridges. This displacement is often carried to 

 such a degree that the three real cardinal teeth are crowded into the 

 anterior half of the hinge-plate, while the accessory teeth occupy the 

 expanded posterior half. 



These accessory teeth do not a])pear to be survivals of primitive 

 embryonic ridges, for they are often less definitely developed in 

 young shells than in adult specimens, and an examination of their 

 number and position in many different species has convinced me that 

 they are developed out of the original nymphal rugosities by the 

 formation of parallel grooves and ridges on this plate. 



1. TlVKLA. 



The simplest form of hinge in Tiveia is exemplified in that of 

 2\ ponderosa, Koch. In the right valve of this species the three 

 ordinary divergent teeth of the Meretricine group are easily 

 distinguishable, the anterior tooth being very narrow and slender 

 and close to the lunular border, the median thick, solid, and 

 triangular, and the posterior narrow and grooved at the top. 

 Bej'ond this and below tlie ligament the nymplial plate is tliickened 

 and rugosely striated, but is not raised into any tooth-like projections. 

 In tlie left valve the teeth are widely divergent and the posterior 

 cardinal occupies its normal position, but its upper and outer side is 

 rugose, as also is the nymph above it. 



