112 Annals of the South African Museum. 



specimen from Addo Drift (46h). For the same reason it is uncer- 

 tain whether the young individual figured by Tate really represents 

 the true T. van. Tate's figure, which represents the specimen in 

 twice the natural size, unfortunately does not give a very satisfactory 

 picture of the youthful characters of either of these shells. 



The close agreement in the characters of the neanic stage reveals 

 the near relationship of these two forms, and, indeed, such a rela- 

 tionship might reasonably be inferred from a comparison of adult 

 characters, which coincide in a remarkable manner as regards the 

 ribbing of the flank and the posterior elongation. Other shells, 

 however, which when adult exhibit similar peculiarities of form 

 and ornamentation, differ widely from T. vau and its associate 

 when in the youthful stage. These are the members of the 

 Trigonia v-scripta group in the marine Oomia beds of Cutch,* 

 and a comparison of their youthful characters and those of T. vau 

 and T. stowi is given in the remarks which are appended to the 

 description of the latter. T. stowi when adult approaches closely 

 in many particulars to T. v-scripta itself, but the adult T. vau agrees 

 much more closely with T. dubia Kitchin, so far as the unsatis- 

 factorily preserved Indian specimens allow of comparison. In 

 consequence of imperfect preservation, the nature of the sculpture 

 in the youthful stage of T. dubia is not known, but it appears 

 reasonable to assume that it bears closer relationship to the other 

 members of the group of T. v-scripta than to T. vau. In the adult 

 stages, too, points of distinction between T. dubia and T. vau are 

 not wanting. These have been set forth in my account of the 

 characters of T. dubia t as follows: "In T. vau, the ribs of 

 the anterior series, though narrow and crowded as in T. dubia, 

 are seen to be upwardly inclined as they depart from their point 

 of meeting with the posterior series, and are crossed obliquely by 

 the lines of growth. In T. dubia, on the other hand, these anterior 

 costae are placed in a direction quite, or almost, parallel to the 

 inferior border, and the angle of the lateral V is therefore not 

 so acute. Moreover, before individuals of T. vau have reached 

 half their full dimensions, the costae of the frontal series cease to 

 be formed close to the frontal border. There arises, therefore, a 

 space devoid of ornamenting ribs, marked only by ridges and 

 furrows of growth, and bounded posteriorly by the last-formed 

 obliquely directed costae of the frontal series. This peculiar 

 character is totally absent in T. dubia, in no specimen of which 

 is a frontal unsculptured space seen, although in respect to size 

 * Kitchin (1), p. 65. f Ibid., p. '69. 



