OF SOUTHERN INDIA. 57 



Mactra (Sciiizodesma) tripartita, Sowerbij, sp., Pi. V, Figs. 8-11. 



1846. Mactra tripartita, Sow., MSS., apud Forbes, Trans., Geol. Soc, Loud., 2 ser., VII, p. 142, pi. 15, fig. 17. 

 Cardium? intersectiim, Forbes, ibidem, p. 145, pi. 18, fig. 8. 



M. testa siihtrigona,mar(jme inferiori moderate curvato instriicta, siihceqiiilaterali, 

 parte posteriori paiilo longiori quam autlca ; umbonibus acutis, antice versus incurvis, 

 distantibus ; superficie costulis, rotundatis vel in junioribus striis approximatis 

 concentricis induta ; declivitate antica ac postica striis pereminenter acutis ac dis- 

 tantioribus ornata, antica sulco angusto infra angulum Itmulareni sito, postica sulco 

 multo latiori supra angulum arealem rotundattim sito separatis. Cardo regtdariter 

 sicut in speciebus recentibus forniatus est, dente cardinali antico in valca sinistra 

 fortasse crassiore. 



Height of sbell to its length 

 Thickness „ ,, 



Young specimens of this species have the anterior side more angular and 

 somewhat longer than the posterior ; old grown specimens hecome, however, nearly 

 equilateral. The distinction hetween the concentric thick ribs or strise on the 

 middle surface of the shell and the thin and sharp ones on the anterior and posterior 

 declivities is very apparent and gave origin to the name of the species. The 

 anterior declivity is separated from the outer or middle surface by a narrow groove 

 situated below the angle which bounds the lunula ; the posterior declivity forms 

 with the middle, or lateral, part of the shell an obtuse angle, and this one is 

 .separated from the latter by a broadly shallow groove. As regards the form, 

 I must remark that, as the edge of the anterior side is thinner than the posterior, 

 it more easily wears away, and that on this account the valves sometimes appear 

 more equilateral than they otherwise would be ; fig. 9 b represents such a specimen 

 which to all external appearance appears perfect, while it certainly is a good deal 

 worn away on the front side. 



The hinge is regularly formed, the cardinal tooth of the left valve is somewhat 

 longer than in most of the recent species, the latei'al teeth and corresponding 

 grooves are of great length, and in both valves mostly prominent in the middle. The 

 ligament lies marginally, just at the end of the cartilage-pit, from which it is not 

 separated by a shelly lamina, the only character which distinguishes Schizodesma 

 from true Mactra. 



Forbes' figure (quoted above) is not quite exact, the original specimen being 

 longer than represented in that figure. 



D'Orbigny was the first to suggest placing the form described by Forbes as 

 Cardium intersectum under Mactra. I have examined Forbes' original, in 1867, 

 and found the specimen very defective, but to all appearance a Mactra ; it is only a 

 little shorter than M. trijmriitaj-eYideutly from being a little worn off- ; and as the 

 ornamentation is in both the same, I was led to consider both as belonging to one 



p 



