52 miss j. donald on the gentjs aclisina. [feb. 1898^ 



Description op the Eeitish Species. 



AcLisiNA PULCHRA, Do KoD., vai. TENiTis, Do Xon. (PL III,- 

 figs. 1-5.) 



AcUsina pulcJira, L. G. De Koninck, 1881. ' Faune Calc. Carb. Belg.' Ann. Mus. 

 Eoy. Hist. Nat. Belg. vol. vi, pt. iii, p. 87 & pi. vii, tigs. 26 & 27. 



Murehisonia tenuis, L. Gr. De Koninck, 1883, ibid. vol. viii, pt. iv, p. 22 & 

 pi. xxxii his figs. 3 & 4. 



Description. — Shell small, conical, composed of from eight to ten 

 whorls. Protoconch irregular, consisting of about one complete 

 smooth revolution, detached from the spire on the umbilical region, 

 and standing nearly erect ; a faint rib marks its junction with the 

 conch, and the ornamenting threads begin almost immediately. 

 Whorls convex below, rather flat above, ornamented by from one 

 to three somewhat fine raised threads on the upper third, and by 

 from three to five stronger threads on the lower two-thirds, with 

 numerous additional finer threads below on the body-whorl, some 

 of which are occasionally visible on the spire below the stronger 

 threads. The spaces between the threads vary slightly in width 

 on different individuals. The lines of growth show the strongly 

 sigmoidal form of the outer lip, which recedes above and comes 

 prominently forward below. Aperture ovoid. Inner lip reflected 

 on the body-whorl, where it forms a thin shelly layer. Columella 

 nearly straight, but curving round below to meet the outer lip.- 

 Umbilicus closed. 



Remarhs. — The actual specimen figured by De Koninck as the type 

 of Aclisina pulcJira in the Brussels Museum is much larger than any 

 of the Scottish shells that I have seen, but approaches them nearly 

 in general form and in ornamentation. The figures and description 

 of De Koninck represent the various ornamenting threads as equal 

 in strength, whereas in reality on the lower part of the spire the 

 three upper threads are finer than the four lower ones and there 

 are several additional fine threads on the body-whorl ; on the higher 

 part of the spire only one of the fine upper threads is present, 

 situated just below the suture. 



The collection of M. le Chanoine de Dorlodot contains an indi- 

 vidual of still greater size, whose aperture is almost entire and the 

 form of the outer lip is seen to accord with the lines of growth on 

 the British fossils. 



Murehisonia tenuis, De Kon., appears to be a small variety of 

 this species, for the contour of the whorls and the ornamentation 

 agree with that of the upper part of the spire of A. pulclira. The 

 specimen figured by De Koninck as the type has only three strong 

 keels on the lower part of the whorl and a single fine one above,, 

 immediately below the suture. The different examples on the tablet 

 in the Brussels Museum, however, show the same variation in the 

 strength and disposition of the keels as the Scottish shells, and 

 they also agree more nearly in size. 



Since the specimens from Scotland bear a closer resemblance to 

 M, tenuis than to the typical A, puUhra, I consider it advisable to 



