13*2 Cuni ings — Morphogenesis of Platystroph ia. 



plications is suggested by the fact that most shells with long 

 hinge-lines and high areas, or any other conformation of the 

 shell indicating a short pedicle, close appression to the surface 

 of support, and consequent interference with the mobility of 

 the valves, are plicate. This is notably true of the Sjnriferacea 

 and Terebratidacea. On the other hand, most shells with 

 prominent ventral beak and apical foramen (many Athyridm 

 and TereoratulidcB), a conformation indicating great freedom 

 of movement of the valves, are non-plicate. Among Atremata 

 and Neotremata, where there is often very great mobility of 

 the valves, plications are lacking ; and as has been recently 

 pointed out by Morse* the setse are here well developed and 

 serve the purpose of strainers. In articulate brachiopods the 

 setae are usually small or aborted. A possibly analogous corre- 

 lation is seen in the Pelecypoda between the development of 

 plications and siphons. When the siphons are well developed 

 and control the incurrent and excurrent water, the shell is 

 seldom plicate. On the other hand, many of the shells which lack 

 siphons are strongly plicate (Pectinacea, Ostracea, Pteriacea^ 

 Arcacea, etc.).j- There is in both brachiopods and pelecypods 

 a tendency to improvise tubes for the passage of incurrent and 

 excurrent water by throwing the margins of the mantle into folds 

 or wrinkles. Morse (loc.cit.) has pointed this out in the case of 

 Lingula lepidula, and N. YatsuJ in that of L. anatina. Some 

 fossil shells suggest the same thing ( Pentamerus, Peptcejia ) .§ 

 Prof. P. T. Jackson informs me that the same is true of some 

 asiphonate Pelecypoda. This tendency may have had some- 

 thing to do in both classes with the initiation of plications. 

 There is undoubtedly also a relation between habitat and 

 plication. It is well known that oysters grow smooth or pli- 

 cate according to the nature of the bottom and purity of the 

 water. AVoodward|| has pointed out an analogous relation be- 

 tween shell ornamentation and habitat among the Brachiopoda. 

 Prof. H. S. Williams*' believes that plications in the Brachio- 

 poda are due to accelerated peripheral growth of the mantle. 

 This is rather a restatement of fact than an explanation of it. 

 In some brachiopods the plications of the nepionic shell cor- 

 respond accurately with the setae.*"* The prolongations of 



* Mem. Bos. Soc. Nat. Hist., v, No. 8, 1902. 



f I am aware of the fact that even in asiphonate Pelecypods, -water is not 

 necessarily admitted along the whole margin of the mantle but is more or 

 less confined to definite parts of it. See Reiny Perrier. Elements d'Anatomie 

 Comparee, p. 661. 



% On the habits of Japanese Lingula, Annotationes Zool. Jap. iv, pt. 2, 

 1002. pp. 63, 64. 



£ Davidson, Sil. Brachiopoda, 1869, pi. xxxix, fig. 4. 



J] Quoted by GEhlert in Fischer's Manuel de Conch vliologie, p. 1243. 



• Geol. Biol., p. 309. 



** See Morse, Mem. Bos. Soc. Nat. Hist.,ii. pt, i, No. 2. 1871, pi. 1, fig. 26. 



