OCR Text |
Show 1866.] MR. W. T. BLANFORD ON OPISTHOSTOMA. 449 verticalis, subtrigonali-rotundata; peristoma simplex, undique expansum. Operculum normale. Diam. incl. peristom. 1*5 mm.; perist. non inch 1 mm.; alt. 1*5 mm.; aperturae diam. circa 0*5 m m. Hab. prope Khandalla ad summos montes "Syhadri" sive Western Ghats appellatos, inter Bombay et Poona, Indiae orientalis. The animal was very difficult to observe, on account of its extreme shyness and minute size. Only a very small portion of the body was extruded from the shell. The foot is very short and apparently rounded, but could not be seen fairly, as the animal would not crawl up a glass but appeared to endeavour to hide itself amongst decayed leaves. The tentacles are short and blunt, the eyes at their outside base, rather high in position, but not nearly so much so as in the Aciculacear. The whole animal is white and translucent, the eyes appearing as black specks, perfectly sessile. After two or three failures I succeeded in examining an operculum by breaking back the whorls of a specimen carefully until I came to it. It is lodged at the constriction in the last whorl, as long since suggested by Mr. Benson, and is distinctly horny, concentric, and paucispiral, resembling the figure of the operculum of Diplommatina folliculus given in Adams's ' Gen. Rec. Moll.' This entirely confirms the views I long since expressed as to the close affinity of Opisthostoma to Diplommatina, and shows the former, moreover, to be nearer to the typical costulate Diplommatinar of the Himalaya than to the smooth or spirally Urate species (Arinia, H. & A. Ad.) of Hindustan, as the latter have the spiral structure apparently obsolete*. It is worthy of remark that some of the Pupinidae (e. g., Pupina artata, Bens.), when their opercula are examined by transmitted light, show an apparently paucispiral structure, due to the whorls increasing in size more rapidly near the centre; but the construction of the spiral in Pupina and Cataulus has been shown to be different from that which obtains in other forms of operculated land shells. The characters which serve to distinguish Opisthostoma fairbanki from O. nilgiricum are :- 1. The greater exsertion and smaller excentric deflection of the two apical whorls of O. fairbanki. 2. The simple expanded peristome and subtrigonally rounded aperture-0. nilgiricum having a non-expanding duplicate peristome, the outer portion retro-relict, and a circular aperture. 3. The more distant sculpture. 4. The manner of curvature of the last whorl, the posterior bend of which is much more acute in the present species. In 0. nilgiricum the posterior half of the sigmoid curve of the last whorl is more open than the semicircular curve nearer the aperture, the umbilicus being fully exposed within the former. In O. fairbanki the anterior curve is the more open, and the last whorl just in front of the pos- * I have just cut out the opercula of two specimens of Diplommatina Bens., from Darjiling, and find the spiral structure much less distinct than it usually is in the Cyclophoridae. |