part 4] THE EYOLUTIOIS^ OF LIA.SSIC aASTROPODS. 301 



' ribs ' mentioned by writers on Liassic Grastropods are transverse 

 to or parallel to the growth-lines. For instance, Tate (even in the 

 same paper) described both types of ornament under the term 

 'ribs.'i 



It will be convenient, therefore, to distinguish clearly between 

 these two styles of ornament ; Mr. Buckman has put forward the 

 two names transcrescent and concrescent, to cover ornament 

 crossing growth-lines and ornament parallel to growth-lines 

 respectively.^ These are applicable to the Brachiopoda and all 

 divisions of the Mollusca. 



(a) Concrescent ornament. — This always follows the lines 

 of growth, and may consist of striae, folds, or costse. In Gastropods 

 it is also conveniently defined as axial ornament, since the ribs 

 are in the direction of the axis of the shell. 



(/3) Transcrescent ornament. — This type of ornament cuts 

 the lines of growth. In Grastropods it may also be defined as 

 spiral ornament, since it follows the spiral of the shell. Primary 

 and secondary (or intercalated) spirals are recognized ; excessive 

 development of one or more spirals leads to carination. 



Either of these styles of ornament may occur independently ; 

 but they are usually associated, and give rise to other types, of 

 which the following are the most characteristic : — 



(a) In which the axials are strong, the spirals merely striae : for example, 



Procerithium slatteri. 



(b) In which spirals are strong, and axials are reduced to fine threads : 



for example, Procerithmm sinemuriense (Martin). 



(c) In which both axials and spirals are equally strong, and form a net- 



work : for exam.ple, Procerithium equireticulatiim, 



(d) In which both axials and spirals are equally strong, but tubercles 



arise at the points of intersection: for example, Procerithium ogerieni 

 (Dumortier). 



Protoconch. — The use of the term ' protoconch ' in papers on 

 Grastropods calls for some comment : the protoconch is not defi- 

 nitely marked oft' by a septum, as in Cephalopods, and so there is 

 considerable difficulty in determining how far it extends. Some 

 authors have applied the term to all the smooth whorls at the apex 

 of the shell ^ ; but these smooth whorls would appear to be com- 

 parable with the several embryonic whorls of Ammonites, and not 

 with the protoconch simply. It thus appears to us that it is more 

 suitable to use the name ' embryonic whorls ' for the smooth whorls 

 of Grastropods, and that the term * protoconch ' should be restricted 

 to the earliest part only of the embryonic whorls. 



1 20, pp. 402, 405. 



2 See Ivor Thomas, op. cit. p. 226. In a letter to us (January 5th, 1921), 

 Mr. S. S. Buckman suggests that transcrescent and concrescent are 

 more appropriate terms than transcrescentic and concrescentic ; he 

 also proposes the term ' obliquicrescent ' for ribs oblique to the growth-lines. 



3 2, vol. i (1895) p. 9; 8, p. 9. 



Q. J. a. S. No. 308. z 



