280 PROCEEDINGS OF THE MALACOLOGICAL SOCIETT. 



that of the Cephalopod in that the greater part of the whorls are 

 exposed, whereas in the latter the older are mostly concealed by the 

 newer whorls. 



The form of the Gastropod shell is multiple, but the types are few. 

 Primitive types always begin with a protoconch having rounded whorls, 

 free from all ornamentation. When the whorls remain rounded 

 throughout the life of the species no regular ornamentation is as a rule 

 produced. Generally, however, a change from the primitive rounded 

 outline to an angular one takes place by the formation of a peripheral 

 keel. The subsequent shape of the shell will then depend upon the 

 position of this keel, whether high up or low down on the whorl, and 

 the extent to which the succeeding whorls overlap each other. 

 Occasionally more than one carina is developed. Keeled shells usually 

 exhibit several forms of surface ornamentation, such as ribs, spiral 

 lirse, nodules, and spines. Of these the ribs are usually the first to 

 appear, but many instances will readily occur to conchologists in 

 which the spiral lines have precedence and are even found on the 

 protoconch itself. The cause of these adornments is uncertain, but 

 Grabau, bettering Ball's suggestion for the columellar plaits of Valuta 

 {17, pp. 58-61 ; 18), attributes the formation of the spiral lirse to the 

 wrinkling of the mantle as the animal withdraws into its shell ; he 

 apparently forgets that the new shell is formed when the animal is 

 extended. Moreover, plications formed simply by the folding of 

 a flexible surface would be apt, like the lines on the palms of our 

 hands, to show considerable individual variation, a condition not 

 exhibited in shells where the regularity of the spiral lirae is usually 

 constant for the species. 



The evolution of the spines and their development in the life of 

 a single individual, as in Murex hrevifrons {26, pp. 934-5) is very 

 happily traced by Grabau, but it is difficult to follow him when, 

 especially in the Melauias, he claims for them a regular phylogenetic 

 sequence {28, p. 639). One wonders what he would make of the 

 ornamentation displayed by Tanalia aculeata (Gmelin), of which 

 H. F. Blanford collected specimens from the same spot in a given 

 stream exhibiting very considerable i-ange of graduated variation, from 

 coarsely granulated to smooth forms {6). Beecher in his admirable 

 memoir (I, p. 353) argues that spines, or, rather, a spinose condition, 

 occurs just after the culmination of a group, and is to be taken as the 

 visible evidence of the beginning of the decline of the vitality of the 

 group. Packard {53, pp. 505-6) inclines to attribute the development 

 of spines, in some cases at all events, to a response to a change of 

 environment, and says: "It is not improbable that the appearance 

 of such highly or grotesquely ornamented forms as certain later 

 Brachiopods, Trilobites, and Ammonites was the result of a change in 

 their environment." In the case of our common Cardiiim ecliinatum, 

 Linn., it may be remarked, the size and number of the spines vary with 

 the nature of the sea-bottom, being fewer and smaller in proportion as 

 the silt in which they bury is firmer. 



Many Gastropods, after passing through the juvenile stage of a plain 

 protoconch and developing the characteristic ornamentation of their 



