PHYLOGENY OF THE PELECYPODA. 



395 



Besides the plates the following figures illustrate the text : 



rigs. 1-8. Ortlwceras elegans and 0. politum, protoconch and apex of conch, by kind- 

 ness of Professor Hyatt, p. 292. 



Fig. 9. Vermetus radicida, adult, p. 294. 



Figs. 10-18. Ostrea edulis, embryology, p. 293. 



Figs. 19-22. " " " p. 297. 



Figs. 23-24. " " " p. 299. 



Fig. 25. " " " p. :W0. 



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



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



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



Figs. 32-34. Avicula sterna, " p. 330. 



Fig. 35. Cassianella decussata, young, . p. 332. 



Fig. 36. Vulsella rugosa, young, p. 332. 



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



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



Fig. 40. Fecten disloratKS, shell structure of young, p. 348. 



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



Fig. 43. 

 Fig. 44. 



Anomia aculeata, young, 

 Unio cariosus, glochiiiium, 

 Margarilana undulata, glochidium, 

 Anodon ct/gnea, glochidium, 

 Anodon fluviatilis, glochidium, 

 Margaritana undulata, young, . 

 Anodon fluviatilis, young, 

 Saxicava rugosa, young, 

 Bhombopteria mira, adult. 

 Malleus vulgaris youug. 

 Pinna sp. indet., young. 



p. 361. 



p. 367. 



p. 367. 



p. 367. 



p. 367. 



p. 367. 



p. 367. 



p. 372. 



p. 380. 



p. 383. 



p. 385. 



Figs. 9, 37, 41 and 42 are electrotypes furnished by the kindness of Professor A. E. Vcrrill. Figs. 10 to 24 and 26 to 29 

 inclusive are electrotypes furnished by the kluduess 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; m, 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 : b, blastopore (after 

 Patten), pp. 288, 291. 



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



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

 /.foot. p. 291. (Figs. 5-6, after Lankester.) 



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 hinge line, and the remarkable spurs, sp, on its ventral border. 

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



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

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



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

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



Fig. 11. Developing il/ontecuta/en'H(/i)iosa, phylembryo stage: ad, anterior adductor muscle, which alone exists at this 

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

 line. (Figs. 9-11 after Lov6n.) Figs. 9-11, pp. 280, 289, 291, 312, 375. 



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

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



Fig. 13. Develojiing 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. : i7, velum; .<!, shell. (Figs. 13-14, after Laca/.e-Duthiers.) pp. 289, 293. 



Fig. 15. Youug Ouniatites compressus : p, ovoid protoconch, succeeded by true couch.' pp. 291, 311, 375. 



'From Hyatt (27) after SaiiUbeigui-. 



