Sjpira I 



Sutural 



angle. 



angle. 



14° 



109° . 



part 4] THE EA^OLUTION OF LIASSIC UASTROPODS. 335 



Katosira triyia (Tate). (PL XXII, fig. 2.) 



Chemnitzia tyivia Tate (22, p. 8). 



Cf. Turritella costifera Piette (13, p. 205 & pi. x, fig. 14). 



Ct". Scalaria liassica Quenstedt (15, pi. xix, fig. 12). 



Dimensions of lectotype (estimated). 



Length. Sreaclth. Length of spire. 



12 mm. 22 pev cent. 76 per cent. 



Lectotype : A specimen in the Museum of Practical Geology, Jermyn 

 Street, London, No. 7990. 



A veiy slender shell, with slightly-curved, rather oblique axial 

 ribs, and fine spirals. The mouth is not distinct in the lectotype ; 

 but in a topotype it appears to be holostome and ahiiost circular. 



Development. — The early whorls are fairly flat, but later 

 whorls become slightly convex, while the last whorl in the specimen 

 figured becomes feebly ridged (convexifastigate). The apical 

 whorls are not present, and the ornamentation described above is 

 characteristic of the whole shell. 



On account of its slender form and the obliquity of its axial ribs, 

 IC. trivia is not likely to be confused with any other species that 

 has been found in England. Some other forms of Katosira with 

 oblique ribs have been described by Piette from the lowest zones 

 of the Lower Lias in France ; one of these is K. costifera (Piette), 

 which is somewhat stouter than our specimens. Catagenetic 

 descendants from these obliquely ribbed species seem to include 

 Turritella intermedia Terquem & Piette, and T. semiornata 

 Terquem & Piette. 



Locality and horizon. — ^The lectotype is a specimen from 

 the Lower Lias of Bridgend ; as the lowest zones only of the 

 Lower Lias are present here, this species is not later than the 

 semicostatus zone, and is almost certain h" from the httcMandi zone. 

 The comparable species found in France appear likewise to be 

 characteristic of a low horizon. 



Katosira (Eigauxia) nogtjesi (Dumortier). (PI. XXII, fig. 3.) 



Chemnitzia noguesi Dumortier (5, pt. 2, 1867, p. 183 & pi. xlv, figs. 4-5). 

 Cf". Turritella undtdata Quenstedt (15, pi. xix, fig. 14). 



Dimensions of specimen figured by us (estimated). 



Length. breadth. Length of spire, 



30 mm. 14 per cent. 85 per cent. 



Kather large but very slender shells ornamented with stout ribs, 

 Avhich do not quite extend from suture to suture, and numerous 

 regular spirals ; the sutural region is wide and shallow, the whorls 

 imbricating. 



Development. — This species shows in the convex early w^horls 

 widely-spaced strong ribs crossed by spirals ; but in the later 

 whorls the axials have become low oblique folds crossed by regular 



2 B 2 ' 



Spiral 



Sutural 



angle. 



. angle. 



10° 



110° 



