A GROWRETA GEMMA. 2 oo ac totapointalo ease = sah coe a ere he eete = oniee eieae aia nee Sere eee a eee eee ee 17 
Fig. la. View of the posterior side ee the ventral valve, enlarged to six diameters. 
1b. Cast of the interior of the apex of the ventral valve, showing a cast of the sipho- 
nal tube and the elongate muscular scars, enlarged to six diameters. 
lc. Interior of the dorsal valve of Obolella? Scabrine Callaway (after Davidson). 
1d. Interior of ventral valve enlarged to six diameters. 
le. Exterior of a small dorsal valve, enlarged to six diameters. 
1f. Interior of apex of ventral valve. 
See plate ix, figs. 9, 9a. 
OBOLELLATAMBIGUA 2 - == <2oc onan deen ne nes as ee se ae ee een en ne ae eee ae alate aera eee 67 
Fig. 2a. Dorsal valve with the central portion of the shell exfoliated so as to show the 
median muscular sears, enlarged to three diameters. 
2b. Exterior of ventral valve, enlarged to three diameters. 
2c. Interior of dorsal valve, enlarged to three diameters. 
SCHIZ/ARUR ON MISE IC ANGI la elma tale alee ere te ee ol aera 70 
Fig. 3. Interior of ventral valve, enlarged to six diameters. 
3a. Exterior of ventral valve, enlarged to six diameters. 
36. Interior of dorsal valve, enlarged to six diameters. 
3c. Exterior of dorsal valve, enlarged to six diameters. 
3d. A portion of the surface of fig. 3c, enlarged to show the minute surface spines. 
NUDDHKOVEENOY COTES: PALME E Gano Song Sooo onan sc CosSon coc nonon one abo omer cson pense oonstocececocice 83 
Fig. 4. View looking from above. Natural size. 
4a. Side view of same specimen. 
MM OTLOTORSIS OC CLO EINE = eter eae ele aaa la lee lle 77 
Fig. 5. Cast of the right valve. See plate xi, figs. 14, 144. 
MODIOLOPSIS Eh OGONIPENSIS oer te tale eee eeltete ate etal cle ate a telat eter 73 
Fig. 6. Cast of the left valve. See plate xi, fig, 13. 
VQ OU VRE SOON IN TES oe Sag o5 Sosseodecas Seer soces Coan Ocoe so esod Ste ses oases serscces 20 
Fig. 7. Upper surface of ventral valve, enlarged to six ee This figure is drawn 
from the type specimen from Montana, described by Mr. Meek. 
7a. Posterior view of the same to show elevation, area, and deltidial opening. 
7b. Enlargement to six diameters of a specimen from the Eureka District. See plate 
ix, fig. 7 
PIB HON OME A UN GIRO TAA eae ere a tere pellet ated tala fe fore eiee eele rea eee 69 
Fig. 8. Interior of ventral valve (after Davidson), Introduced for comparison with the 
ventral valves of Acrotreta and Schizambon. 
IPTYCHOPARTA}? SIMEDIS: ‘Var: ROBUSTIUS) «ose neseae ee ees= ae oe eae eee eos eae ne seseeen aeanee 53 
Fig. 9. View of the central portion of the head. Natural size. 
9a. Outline of profile view of the same. 
DICEETVOCRPBATUS HINER PEO DANG jar mints ea ean ae aie en ine ee ee coe aero ete 90 
Fig. 10. Central portions of the head of a small specimen. Natural size. 
METOPTOMA ? ANALOGA.....-- PC ODOR OA DOS OSES DDO at Ori De SHOAIB SOE ACS e SSH eS soSnoSSOose oan 84 
Figs. 11, lla. Summit and side view of the type specimen. Natural size. 
e 
MURGHIRBONTAUMIMGERD <\o.osts capt eclosion ato alate en ee inte le ecstatic 271 
Figs. 12, 12a. Front and back view of the same specimen. Natural size. 
12b. View of a cast. Natural size. 
BUCANIA BIDORSATA. « .. .--= =~ 200 22 2 en see eir en ens cee mrinwee tees senncece acne santos nonecscecens 271 
Figs. 13, 18a, 13c. Back, lateral, and front view of a specimen denuded of most of the shell. 
Natural size. 
130. Enlargement of the surface of another shell, that shows the characteristic sur- 
facefines. 
