CORALS FROM THE CORAL RAG. 99 
to do away with the nominal distinction between Thamuastrea and Synastrea, and to 
designate all the species appertaining to them under the oldest of the two generical names, 
which is that of Zhamnastrea.' 
The group thus composed contains a great number of species, most of which belong to 
the jurassic or cretaceous formations. Zhamuastrea arachnoides differs from most of them 
by the existence of a basal plate destitute of epitheca; but it bears great resemblance to 
some cretaceous corals, and more especially to Z. agaricites,? T. cistela, T. conica,* and 
T. decipiens.’ It differs, however, from the first of these by the calices being smaller and 
the septa more numerous ; in 7” ciste/a the septa are, on the contrary, more numerous than 
in 7. arachnoides ; in T. conica the septa are thinner, besides the general form being very 
different ; and in 7. decipiens the septa are thicker and form only three cycla. Compared 
to the jurassic species, 7: arachnoides may be distinguished by similar peculiarities ; thus 
in 7. concinna® the calices are much smaller, and the septal systems more simple ; in 
7. scita’ the septa are more delicate and closer set; 7. Lyelli,’ 7. dendroidea,? T. mammosa," 
T. Waltoni™ T. cadomensis,” and T. afinis,” ave of a dendroid or mammilose form; in 
T. Defranciana* and T. mettensis'’® the septa are again thinner, and in 7! Zerguemi’ they 
are, on the contrary, more robust ; in 7. fungiformis they are more numerous and more 
strongly denticulated. As to most of the new species mentioned by M. D’Orbigny,’* their 
characters have not been pointed out with sufficient minuteness to enable us to distinguish 
them from the fossil described in this chapter. 
1 In so doing, we must, however, remark that one of the fossil corals of the Neocomian period, the 
Astrea micrantha of Roemer appears to have a real styliform columella highly developed, as well as septa 
with entire edges, characters which we erroneously attributed to Thamnastrea, If that be really the case, 
this species must constitute the type of a distinct genus, to which the name of Holocenia may be given. 
The genus Centrastrea of M. D’Orbigny, (Note sur des Polyp. Foss., p. 9,) does not contain Astrea mi- 
crantha, and the species referred to this division in that naturalist’s ‘ Prodrome’ do not, in reality, differ from 
Thamnastrea, their supposed prominent columella being adventitious. We also see no sufficient grounds 
for adopting the genus Polyphyllastrea of M. D’Orbigny, the species for which it was established differing 
from Thamnastrea only by having a greater number of sepéa than is commonly the case in those corals. 
2 Astrea agaricites, Goldfuss, Petref. Germ., t. 1, pl. xxi, fig. 9. 
3 Astrea cistela, Defrance, Dict. Se. Nat., t. xlii, p. 388. 
4 Astrea conica, Defrance, Dict. Se. Nat., vol. xlii, p. 387. 
8 Astrea decipiens, Michelin, Icon. Zooph., pl. xe, figs. 12, 13. 
® Tab. xviu, fig, 3. 7 Tab. xxii, fig. 4. So Daby xxi, eed. 
9 Astrea dendroidea, Lamouroux, Exp. meth., pl. Ixxvii, fig. 6. 
10 Tab. xxiii, fig. 3. U Tab. xxix, fig. 4. 
12 Astrea cadomensis, Michelin, Icon., pl. xciv, fig. 4. 
13 Milne Edw. and J. Haime, Ann. Sc. Nat., 3™° ser., vol. xii, p. 198. 
14 Tab. xxix, figs. 3, 4. 1b ab. xxx, fig. 3: 16) Tab. xxx, fig. 2: 7 Tab. xxx, fig. 4. 
18 Tab. xxx, fig. 3. 
