332 MISS MCDONALD a:nd de. TiiUEMAiSr ON [vol. Ixxvii^ 



Development. — Apical whorls of shell not preserved; the 

 earliest whorls present are convex, somewhat tmnid, and ornamented 



by feeble ribs; the last three whorls 



Fig. 20. — Zygopleura semi- are smooth, except for shar^Dly 



tecta (Tate). Holotype. curved growth-lines, and perhaps 



Diagram sJioicing curve exceedingly faint spiral lines. A 



of growth -lines. well-marked subsutm-al band is 



present, which may be formed by 

 a slight tendency to mould the pos- 

 terior part of the whorl on the 

 preceding whorl. 



This species resembles other 



Loxonematid species found at 



various horizons in the Lias, some of 



which have been referred to the 



genus Anoptycliia by Dr. Cossmann. It differs from these species 



in the convex whorl and in the curve of the growth-line, and is a 



homoeomorph of such forms as Zygopleura liemicolpa. 



Locality and horizon. — The holotj^pe is from the Middle 

 Lias {niargaritatus zone), Staithes, Yorkshire. 



(B) Katosiea. 



Katosira is, on the Avhole, more abundant in the Lower Lias 

 than Zyqopleura, and extends into the Middle and LTpper Lias. 

 The evolution of Katosira may be considered to be j^arallel to 

 that of Zygopleura. for man}^ of the lower forms, generally 

 speaking, have fewer axials (these may include Cliemnitzia 

 etalensis Piette and Cli. collenoti Terquem & Piette). The 

 number of the axials is increased as the series advances, and at the 

 same time there is a tendenc}^ for them to become more sharply 

 curved ; the spiral ornament is at first spread even]}^ over the 

 whorl, but in later forms it tends to become more prominent near 

 the sutures. In exceptional cases, the spirals are sufficiently 

 strong to produce a suggestion of tuberculation. In some 

 forms, the whorls embrace in the manner noticed in some species 

 of Zygopleura. 



Katosira cf. youngi (Tate). (PI. XXII, fig. 5.) 



Cliemnitzia youngi Tate (24, pi. xvi, fig. 6). 



Turbo youngi Simpson (18, p. 156). 



Cf. Cerithium quinetteum Terquem & Piette (26, pi. v, fig. 4). 



Cf. Zygopleura {Katosira) cJiartroni Cossmann (3, pi. ix, fig. 57). 



Cliemnitzia hlainviUei Kicliardson nan Miinster (17, pi. ii, fig. 8). 



Dimensions of our sj)ecimen. 



-^ ,, -r> 7J.7 T J.1 r • Spiral Svtural 



Length. Breadth. Length of spire. f^^^^^^ ^^^^^ 



12 mm. 25 per cent. 70 per cent. 22° 108° 



A slender form having convex whorls ornamented with slightly 

 curved ribs, crossed by numerous fine spiral threads which are more 

 pranounced near the sutures. Aj^erture incomplete. 



