258 MISS JANE DONALD ON THE GENEEA [j^tlay 1 899, 



the enter lip retreated greatly towards and advanced still more 

 strongly from the sinns, and also the possession of a sinus shallower 

 than that of the typical Murchisonice, which have a slit bordered 

 by parallel edges. The surface of the whorls is smooth or merely 

 ornamented by fine threads, with the exception of the frequent 

 occurrence of a very strong thread or swelling immediately below 

 the suture. The position of the band varies in different species, 

 being near the middle of the whorl in the type, but considerably 

 below it in H. antiqua and H. cingulata, His., while in others it is 

 intermediate in situation, 



Ulrich & Scofield consider the characteristics so distinctive as to 

 mark out Hormotoma as a separate genus, instead of merely a section 

 of Murchisonia, and I think it advisable to follow them in this 

 matter. 



Resemblances. — In general appearance this genus most nearly 

 approaches Ccdocaulus^ (Ehl.,^ from which it is distinguished by 

 its less elongated form, more convex whorls, deeper sinus, more 

 strongly retreating and advancing lines of growth, and aperture of 

 different shape. It also bears some resemblance to Aclisoides, Don.,^ but 

 differs in having more oblique lines of growth, a very slightly defined 

 band, and in the ornamentation, when present, not consisting of such 

 strong raised threads and keels. Ulrich & Scofield believe that the 

 relationship of Hormotoma lies between Plethospira, U. & Sc, (type ffo- 

 lopea cassina Whitf.), and Turritospira, U. & Sc. (ITurritoma, U. & Sc). 

 On p. 1013 (op. cit.) Ulrich & Scofield speak of Hormotoma being 

 related to Turritospira, Ulr., though they do not appear to describe a 

 genus under that name, but under Turritoma (p. 959), which genus 

 they also state is related to Hormotoma. Are the two names intended 

 to designate the same genus? and if so, which is to stand? 

 M. aerea, Billings, is given as the type of Turritoma, and it 

 certainly seems unfortunate that the above-mentioned authors have 

 chosen as type of a new genus a species in which none of the surface- 

 markings are represented or described. From Pletlwspira it is 

 distinguished by having a deeper sinus in the outer lip, and in being 

 more elongated. In Turritoma the contour of the whorl is different, 

 being flatter or slightly convexo-concave above and prominent below, 

 where the band is situated ; the character of the lines of growth is 

 not indicated. 



Eange. — In the British Isles this genus is known first to occur in 

 strata referred either to the Upper Cambrian, or else to rocks near 

 the base of the Ordovician, audit continues from this horizon upwards 

 throughout the Silurian. The earliest British forms occur in the 

 Durness Limestone, and the latest in the Upper Ludlow Kocks. I 

 am not acquainted with any Devonian or Carboniferous representa- 

 tives in the British Isles. Salter, in his ' Catal. Cambr. & Silur. 



^ I am gveatly indebted to M. and Mme. Q^blert for lending me C Davidsoni 

 (figured in Bull. Soc. Etudes sci. Angers, 1887, p. 20 & pi. vii, figs. 4:~id), 

 the type of the genus, and. I was thus enabled to couipaie it with members of 

 Hormotoma. 



2 Quart. Journ. Geol. Soc. vol. liv (1898) p. 66. 



