268 i>R. C. T. TRECHMANN OIS T THE [vol. lxxix, 



with the left valve, although the beak is less swollen and does not 

 project above the hinge-margin. Owing to the damage to the 

 beak, the ear of the right valve cannot be seen. The shell is thin, 

 but its structure is not ascertainable, as it has been replaced by 

 hsematitic material. 



Dimensions. — Length = 57 mm.; height at the beak = 25 mm.; 

 towards the posterior margin = 31 mm. Thickness of valves 

 = 20 mm. 



Locality. — Waikato, South Heads. A specimen with both 

 valves in apposition, the umbones rather damaged. 



Remarks. — The specimen just described differs from A. spiti- 

 ensis var. extensa in the more regularly rounded surface of the 

 left valve, the less swollen and less arched beak, the fact that the 

 shell broadens out posteriorly more rapidly, and in the possession 

 of foliaceous growth-ridges. It seems to agree nearly, though not 

 exactly, with the left valve shown in Hoklhaus, pi. xcvii, tig. 8, 

 although in that specimen the beak seems to be more anterior, 

 and the outline to widen out more rapidly. 



If one had only the New Zealand material to deal with, one 

 might feel inclined to place these two Aucellce in different 

 species ; but, from a study of the forms of Aucella described as 

 A. spitiensis, it would appear to be unnecessary to do so. 



Dr. Karl Hoklhaus describes three 'varieties' of A. spitiensis, 

 in addition to the typical form, namely, vars. extensa, grandis, 

 and superba, which he unites as ' sjmchronous variations of a 

 single species '. I have very little doubt that the elongate 

 Aucellce from Waikato are referable to the Himalayan species of 

 the Spiti Shales. Two other species from Spiti are described in 

 the same memoir: namely, A. leguminosa Stoliczka and A. blan- 

 fordiana Stoliczka, and it is remarked that both A. leguminosa 

 and A. spitiensis are related to A. bronni Rouillier, but are at 

 the same time perfectly distinct; while A. blanfordiana is closety 

 related to A. pallasi Ke} r serling. In Russia a considerable ver- 

 tical interval separates A. bronni from its derivative, A. pallasi. 

 but the Indian specimens were not collected with sufficient care 

 to show whether a series of derivative mutations is represented 

 by them. 



The Himalayan Aucellce seem to yield little information of 

 value in establishing the age of the beds, except that they are 

 Upper Jurassic. Hoklhaus says that a few of the occurrences 

 point to an Upper Jurassic age as the most probable one for the 

 Spiti Shales : for instance, certain species of Aucella the nearest 

 allies of which are the European forms A. bronni and A. pallasi, 

 and also a Trigonia closely allied to several species from the 

 Jurassic of Kach (Chari Group). 



Among some Aucellce recently collected by Mr. J. A. Bartrum 

 at South Heads, Waikato, and kindly sent to me, are two left 

 valves and a right valve of a form indistinguishable from 

 A. blanfordiana. They bear a close resemblance to the form 

 figured by Huldhaus, pi. xcviii, figs. da-Bb & 4?a-4:b. The 



