698 



LAMELLIBRANCHIA TA. 



[STR. TAI. 



Sheets 17th Rep. Geo. Sur. Ind., p. 

 89, Waverly Gr. 



Streptodiscxjs, S. A. Miller, 1892, Advance 

 Sheets 18th Rep. Geo. Sur. Ind., p. 71. 

 [Ety. streptos, twisted ; diskos, quoit.] 

 Proposed instead of Trematodiscus of 

 Meek & Worthen, which was preoccu- 

 pied. Type S. stygialis. 

 indianensis, S. A. Miller, 1892, Advance 

 Sheets 18th Rep. Geo. Sur. Ind., p. 71, 

 Keokuk Gr. 

 ohioensis, Miller & Faber, (Trematodiscus 

 ohioensis,) 1892, Jour. Cin. Soc. Nat. 

 Hist., vol. 14, p. 168, Waverly Gr. 



Tkmnochilus conchiferum T. crassum, and 

 T. depressum, Hyatt, 1891, 2d Ann. Rep. 

 Geo. Sur. Texas, pp. 1529 to 333, Coal 

 Meas. 

 forbesanum instead of Nautilus forbes- 

 anus, on p. 414. 



Tetragonockka8, Whiteaves, L890, Trans. 

 Roy. Soc. Can., vol. 8, p. 105. [Ety. tetra, 

 four; gonia, angle ; keras, horn.] Shell 

 subspiral, recurved, coiled loosely on 

 the same plane, making a lax volution 

 toward the apex, but nearly straight 

 anteriorly; transverse section quad- 

 rangular ; siphuncle cylindrical, exo- 

 gastric, marginal, and placed in the 



middle of the venter. Type T. gracile, 

 which is described at the same place 

 from the Devonian. 



Trematodiscu*, see Streptodiscus. 



Trociioceras maccharlesi, Whiteaves, 1889, 

 Trans. Roy. Soc. Can., p. 81, Tren- 

 ton (?) Gr. 



Waagenoceras, Gemmellaro, 1888, <<ior. 

 Sci. Nat. Ed. Econ., vol. 19, p. 11. [Ety. 

 proper name; keras, horn.] Shell dis- 

 coid ; volutions embracing and bearing 

 transverse constrictions ; septa nu- 

 merous and complex. Type W. stachei. 

 cumminsi, White, 1889, Am. Nat., vol. 23, 

 p. 117, and Bull. U. S. Geo. Sur., No. 

 77, p. 20 Permian Gr. 



Tainoceras, Hyatt, 1883. Proc. Bost. Soc. 

 Nat. Hist., vol. 22, p. 261). [Ety. tainia, 

 a head-band; keras, horn.] Whorls 

 discoidal ; section quadrate ; two lateral 

 rows of tubercles, and two rows on 

 the outer side in the later stages of 

 growth; siphon above the center; 

 sutures have ventral, lateral, and dor- 

 sal lobes, but no annular lobes. Type 

 T. quadrangulum. It is very much 

 like Metacoceras, if not a synonym, 

 cavatum, Hyatt, 1891, 2d Ann. Rep. Geo. 

 Sur. Texas, p. 341, Coal Meas. 



LAMELLIBRANCHIATA. 



This Class has gradually improved in the Geological ages, and is now in the 

 maximum of its development. The lobes of the animal are right and left, and 

 unite along the dorsal or hinge-line of the shell, and extend laterally in the form of 

 mantles to the pallial line. The depth of the pallial sinus indicates the size of the 

 siphonal muscles. The shell opens along the ventral side. The external hinge 

 ligament consists of horny fibres that are found preserved, sometimes, almost as 

 well as the shell, in Lower Silurian rocks. Some shells have an internal ligament 

 or cartilage between the hinges of the two valves, located in pits or cartilage furrows 

 on the faces of the hinge. The anterior adductor is in front of the mouth, and the 

 posterior adductor behind the umbo, near the termination of the intestine. Some- 

 times scars are left by the pedal muscles used for projecting and retracting the foot. 

 When there are no teeth or crenulations on the hinge, the shell is edentulous. The teeth 

 beneath the umbo are the cardinal teeth, and those anterior or posterior to the umbo 

 are lateral teeth. Some shells are composed of aragonite, others of calcite, and others 

 have an outer layer of calcite and an inner one of aragonite, and generally there is 

 an epidermis or outer horny coating. The outer calcite layer is secreted at the mar- 

 gin of the mantle or circumference of the shell and is prismatic, while the inner 

 layers of aragonite are secreted in the form of laminae by the sides of the mantle. 



The families TJnionidse and Ostreidee, I think, are not palaeozoic. Anthracosia, 

 Prisconaia, and Cardinia might be referred to the Cardiniidse ; Clinopistha and 

 Solenomya to the Solenoinyidse. 



