. Bulletin of the Natural Histort Museum. Geology series. 60 HOWARTH. Fig. 3 Idoceras cf. hanirinum Venzo. whorl cross-sections of CA1034 of PI. 9, fig. 1; A, at 128 mm whorl height and about 430 mm diameter; B, at 114 mm whorl height and 320 mm diameter; C, at 100 mm whorl height and 270 mm diameter; all mostly preserved solid, being slightly crushed on parts of the outer side of the whorl. The shell immediately adjacent to the mouth- border is crushed, except for the ventral rostrum which is intact, and the short part of the end of the phragmocone is crushed. The whorl section ha
. Bulletin of the Natural Histort Museum. Geology series. 60 HOWARTH. Fig. 3 Idoceras cf. hanirinum Venzo. whorl cross-sections of CA1034 of PI. 9, fig. 1; A, at 128 mm whorl height and about 430 mm diameter; B, at 114 mm whorl height and 320 mm diameter; C, at 100 mm whorl height and 270 mm diameter; all mostly preserved solid, being slightly crushed on parts of the outer side of the whorl. The shell immediately adjacent to the mouth- border is crushed, except for the ventral rostrum which is intact, and the short part of the end of the phragmocone is crushed. The whorl section has a smoothly rounded umbilical wall and edge and an evenly rounded venter. The sides of the whorl converge strongly towards the venter and the greatest whorl breadth is a quarter to a third of the whorl height from the umbilical seam. The venter of the previous whorl can be seen at five breaks in the body-chamber (Fig. 3); it is evenly rounded like the venter of the body-chamber, but on the smallest visible cross-section the inner whorl (ie. about whorls before the mouth-border, at a diameter of approximately 140 mm) has a narrower rounded venter and flatter sides, like that of a more compressed Idoceras. Small portions of the final septum are also visible here, attached to the outside of the venter of the inner whorl. The shell is visible at several places on the body-chamber, and is completely smooth, with no trace of any ornament. Measure- ments of CA1034: at 460 mm diameter: 135 (), 97 (), 215 (). Remarks. This very large body-chamber closely resembles the largest of Venzo's (1959: pi. 7, fig. 2) figured specimens, which is here designated lectotype. That specimen is 280 mm diameter, and appears to be a complete adult macroconch with whorls of body-chamber. It has traces of forwardly angled secondary ribs on the venter at the end of the phragmocone, whorls before the aperture, and appears to become smooth from the beginning of the body-chamber. The
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