PHYLOGENY OF THE PELECYPODA. 395 



Besides the plates the following figures illustrate the text : 



Figs. 1-8. Orthoceras elegans and 0. politum, protoconch and apex of conch, by kind- 

 ness of Professor Hyatt, p. 202. 



Fig. 9. Vermelus radicula, adult, P- 291. 



Figs. 10-18. Ostrea edulis, embryology p. 205. 



Figs. 10-22. " " " P- 207. 



Figs. 23-24. " " " p. 209. 



Fig. 25. " " " P- 300. 



Fig. 26. Oslrea edulis, prodissoconch p. 312. 



Figs. 27-29. Ostrea edulis, the spat, P- 314. 



Figs. 30-31. Perna ephippium, young, p. 329. 



Figs. 32-34. AvicJila sterna, " P- 330. 



Fig. 35. Cassianella decussa'.a, young, . p. 332. 



Fig. 36. Vulsella rugosa, young P- 332. 



Fig. 37. Pecten irradians, adult, p. 333. 



Figs. 38-39. Pecten magellanicus, young, p. 348. 



Fig. 40. Pecten dislocatns, shell structure of young, p. 348. 



Figs. 41-42. Anomia glabra, adult, p. 354. 



Fig. 43. Anomia andeata, young, P- 361. 



Fig. 44. XJnio cariosus, glochidium, P- 367. 



Fig. 45. Maryaritana undulata, glochidium, p. 367. 



Fig. 46. Anodon cygnea, glochidium, p. 367. 



Fig. 47. Anodon flttviatilis, glochidium, p. 367. 



Fig. 48. Maryaritana undulata, young, p. 367. 



Fig. 49. Anodon fluviatilis, young, p. 367. 



Fig. 50. Saxicava rugosa, young P- 372. 



Fig. 51. Bfiombopteria rnira, a.dn\t, P- 380. 



Fig. 52. Malleus vulgaris young p. 383. 



Fig. 53. Pinna sp. iudet., young, P- 385. 



Figs. 0, 37, 41 and 42 are electrotypes furnished by the liindness of Professor A. E. Ven-ill. Figs. 10 to 24 and 26 to 29 

 inclusive are electrotypes furnished by the kindness of Colonel MacDouald, United States Commissioner of Fisheries. 



PLATE XXIII. 



Fig. 1. Embryo of Patella in the morula or mesembryo stage. Fig. la is an optical section of a similar stage (after 

 Patten), p. 291. 



Fig. 2. Embryo of Paludina, gastrula, or metembryo stage: g, gastrula invagination; vi, mesoderm cells (after 

 Butschli). p. 291. 



Figs. 3-4. Embryo of Patella, trochosphere or neoembryo stage viewed from the dorsal side. In the middle of this 

 region the shell gland appears. Fig. 4 is a similar specimen viewed from the opposite or ventral side : 6, blastopore (after 

 Patten), pp. 288, 201. 



Fig. 5. Embryo of Pisidium pusilum, typembryo stage: s, sliell gland; in, mouth; a, anus;/, foot. p. 291. 



Fig. 6. Embryo of Plenrobranchidium, typembryo stage: s, shell gland and beginning of shell; v, velum; m, mouth; 

 /.foot. p. 201. (Figs. 5-6, after Laukester.) 



Figs. 7-8. Glochidium of Anodon fluviatilis, from the gill cavities of the parent : ad, adductor muscle. This period is 

 referable to the phylembryo stage. The shell has a straight liinge line, and the remarkable spurs, sp, on its ventral liorder. 

 Fig. 8 is a dili'erent view of a similar specimen. Medford, November, 1888. X 75 diam. pp. 301, 312, 367. 



Fig. 9. Developing Modinlaria marmorafa, phylembryo stage : ad, anterior adductor muscle, which alone exists at this 

 stage ; m, mouth ; s, sliell, which has a straight hinge line. 



Fig. 10. Developing Cardium exiguum, phylembryo stage: ad, anterior adductor, whicli alone exists at this stage; 

 V, velum; m, mouth; i, intestine; a, anus; s, shell, which has a straight hinge line. 



Fig. 11. Developing Montacutaferruginosa, phylembryo stage : ad, anterior adductor muscle, wliich alone exists at this 

 stage; v, velum; m, mouth; a, anus; /, foot; r, r, r, retractor muscles of the velum; s, shell, whicli has a straight hinge 

 line. (Figs. 9-11 after Loveu.) Figs. 9-11, pp. 280, 289, 201, 312, 375. 



Fig. 12. Adult Slava fibrosa, showing a very marked change in form of the sliell : n, nealogic stage; e, ephebolic stage 

 (after Barraude), life size. pp. 293, 294. 



Fig. 13. Developing Dentalium, phylembryo stage: v, velum; s, saddle-shaped shell, pp. 289, 291. 



Fig. 14. Developing Dentalium, older than fig. 13. The completed periconch stage, comparable to the period of the 

 completed protoconch in Gasteropods, etc. : v, velum; s, shell. (Figs. 13-14, after Lacaze-Duthiers.) pp. 289, 293. 



Fig. 15. Young Guniatites compressus : p, ovoid protoconch, succeeded by true conch.' pp. 291, 311, 375. 



Trom Hyatt (27) after Saudbeiger. 



