214 PROCEEDINGS OE THE MALACOLOGICAL SOCIETY. 



Plectoptlis Messageki, n.sp. PL IX, Fig. 3. 



Shell dextral, very widely umbilicated, discoid, light or dark corneous, 

 finely and regularly ribbed above, becoming smoother below, decussated 

 with very fine spirals. Spire depressed, apex prominent, suture rather 

 deep. Whorls 7, increasing slowly and regularly, slightly convex 

 above, rounded at the side, inflated below, angulated at the upper part 

 of the last whorl, and obtusely subangular round the umbilicus ; the 

 last whorl dilated towards the mouth, shortly deflected in front. 

 Aperture elongate-elliptic, oblique, margins slightly convergent, united 

 by a slightly raised flexuous callus on the parietal wall. Peristome 

 thickened and considerably expanded, but slightly reflexed ; upper 

 margin arcuate, outer and basal rounded, columellar straight. 



Parietal armature composed of two vertical plates, the anterior 

 flexuous, its lower part receding and giving off posteriorly a longish, 

 obliquely descending ridge, and with a slight support anteriorly at the 

 upper extremity, above which occurs a short, free, horizontal fold. 

 The posterior plate is curved, obliquely descending backwards, and 

 with a short posterior ridge at the upper extremity. 



Palatal armature consisting of six folds ; the first nearly horizontal, 

 low and thin, the next four oblique, semicircular, and flexuous, united 

 by a vertical callus ; the second ascending at first, then horizontal, the 

 next three horizontal at first, then ascending ; sixth oblique, semi- 

 circular, but short. Diam. maj. 18-5, min. 14*5 mm. ; alt. 8 mm. 

 Sab. — Muong-Hum. Type in my collection. 

 Diam. maj. 16, min. 13 mm. ; alt. 6 '25 mm. 

 Sab. — Nat-Son, Pac-Kha, and Trinh-Tuong. 

 Yar. minor. — Diam. maj. 12-75, min. 10'75 mm. ; alt. 6 mm. 

 Sah. — Nat-Son. 



This new species resembles P. Iloellendorffi ; but, although more 

 elevated in the spire, the axis is relatively shorter, and the last whorl 

 is much less deflexed, the aperture at the same time being more 

 dilated laterally. In the parietal armature it differs in having the 

 anterior vertical plate produced posteriorly below into a descending 

 long ridge, which, with the anterior support of the upper extremity, 

 gives this structure the form of the letter Z, while the free 

 horizontal fold occurring below it in P. Moellendorjjfi is here absent. 

 The palatal armature also exhibits some differences, notably in 

 the second fold being in the new species much shorter, and having 

 the four folds united by a transverse callus, a feature not observed in 

 P. Moellendorffi. 



A specimen from Pac-Kha shows traces of hybrid origin, for while 

 it has the second palatal fold short, as in P. Messageri, the anterior 

 parietal plate lacks the posterior long ridge below, and has instead 

 a free horizontal fold as in P. Moellendorffi. 



A number of shells from Gia-Phu are smaller than the type, 

 measuring only 16mm. in diameter; they have a relatively higher 

 spire, and have the last whorl more deflexed in front. The species is 

 named in honour of Colonel Messager, who has contributed so largely 

 to our knowledge of the molluscan fauna of Tonkin. 



