part 4] 



THE EVOLLTIO^' OF LIA8SIC GASTEOPODS. 



323 



Fig. 13. — Cerithinella cf. 

 confusii (Tate). 



i. 



Ilk 



u. i^. 



A slender Hat-whorled form, with a low apical angle and 

 characteristic ornamentation . 



Development. — The earliest whorls are not present in om* 

 specimen ; on the first whorl preserved are tliree rows of nodules, at 

 first evenly-spaced and practically equal in size (fig. 13 i), but 

 before the end of the whorl a space has appeared between the two 

 anterior rows ; for several succeeding whorls this style of ornamen- 

 tation is retained, there being several unbroken spiral threads 

 separating the two posterior from the anterior row of nodules. On 



the tenth whorl present in our speci- 

 men the ornamentation is similar ; but 

 the nodules appear to be isolated rather 

 than raised portions of the axials (see 

 fig. 13 ii). In the last whorl the 

 nodules of the second row, counting 

 from the posterior end, become elon- 

 gated along the lines of growth and 

 divide into two ; the base of tlie shell 

 is ornamented with spiral threads (fig. 

 13 iii). 



Our specimen appears to resemble 

 closel}^ a specimen from the same 

 locality figured by Wilson & Crick. 

 It is not, however, so far advanced as 

 the type of Tate's species, in which 

 the posterior portion of tlie whorl is 

 ornamented by five rows of nodules, 

 separated from the two anterior rows 

 by a band with spirals only. It is 

 possible that C. confiisa (Tate) merely 

 represents a more fully grown speci- 

 men than the one that we have been 

 considering ; but in any case the two forms evidently are closely 

 connected, the extra row of nodules having been developed as seen 

 in the development of our specimen. 



Other species of Cerithinella found in the Lias appear to be 

 closely related to these, since most of them are alike in having the 

 granules concentrated in two bands, separated by a zone with 

 spirals only. We have been unable to obtain complete specimens 

 of Cerithinella in which the development can be worked out ; but, 

 from the incomplete specimen of C. cf. confusa, it seems possible 

 that this group of forms has been evolved from more normal forms 

 of Procerithidae. 



Locality and horizon. — The specimen examined, in the col- 

 lection of Mr. J. W. Tutcher (No. 1010), is from the Upper Lias, 

 acutum zone, of Tilton (Leicestershire). 



HI. 



[i= first whorl present in spe- 

 cimen ; ii = eig'lith whorl 

 present ; iii = last whorl, ] 



