APPENDIX. 3 



loop placed between the septa, but it will be sufficient to remind the reader, that it is the exterior margin 

 of the loop-riband which becomes fixed to tbe valve, and that the outer margin must bear the cirri. 



The bridge (PL VI, fig. 37, w, Int.) offers so many striking analogies with the "converging pro- 

 cesses " near the crura of Argiope, that it appears impossible to seek a representation of these parts in any 

 portion of the appareil descendant; so that while describing the brachial arrangement in Thecidium 

 digitatum or T. papillaturn (= radians, Def.), we should say, the loop is forced to form a curved line by the 

 protrusion of the septa (as in Argiope decollata, &c), the exterior side adhering wholly to the valve and 

 septa (as it does partially so in A. decollata), while the converging processes are united, so as to form the 

 bridge, the crura being likewise fixed to the valve. The brachial membrane is protected by peculiar cal- 

 careous supra-membraneal deposits, termed appareil descendant by M. E. Deslongchamps. 1 



It seems, therefore, that M. Suess entirely objects to any portion of M. E. Deslongchamps' last-named 

 appareil representing the loop; that in Th. digitatum, for example, all the branches or grooves are equal, 

 while in Th. papillaturn tbe outer branches of the appareil descendant are both larger and broader than the 

 more central ones ; but that in Th. Mayale, Desl., the supra-membraneal disk appears to be really wanting, 

 while the equivalent of the loop may easily be recognised, forming a raised rim all round the septa. The 

 last-named species would therefore approach nearer to Argiope than any other hitherto discovered, and 

 might, perhaps, deserve to be considered as the type of a distinct group or sub-section of Thecidium ; but 

 M. Suess considers that before arriving positively at such a conclusion, additional researches will require to 

 be made on perfect interiors of Th. rusticum (Moore), where no trace of appareil descendant has hitherto 

 been observed. 



The supra-membraneal disk is always fixed to the valve about the centre of the shell, on either side ot 

 the inferior part of the visceral cavity, but there exists much difference as to the extent to which the 

 disk is fixed. In Th. digitatum and in Th. sinuatum it adheres throughout ; the portion above the visceral 

 cavity only remaining free. 



After these and other preliminary observations, M. Suess enters into an elaborate description of the 

 Thecidium vermiculare, Schl., sp. (= Th. hippocrepis, Goldfuss), and observes that in Th. sinuatum the portion 

 of the supra-membraneal disk above the visceral cavity is not a solid calcareous mass, but a loose net-work 

 fixed to a solid rim, termed the bride transversale by E. Deslongchamps. This remarkable and delicate 

 net-work is beautifully exemplified in Th. papillaturn (= radians), and still more widely open in Th. vermi- 

 culare. In Th. digitatum all the windings of the brachia are nearly on the same line or level ('Introd.,' 

 PL VI, fig. 40 2 ), while in Th. papillaturn they rise from about the middle of the shell, and to such an 

 altitude in Th. vermiculare as to stand nearly vertically to the direction of the valve. 



Thus the characteristic peculiarity in the last-named Thecidium, and a few other allied species, consists 

 in the elevation of this supra-membraneal disk ; it does not become entirely free, but those portions which 

 used to be formed of solid masses in other species, are here represented by thin, delicate net-works. In 

 Th. papillaturn we see contrary to what Th. digitatum shows, that all the branches of the loop and lobes to 

 which they are united have a tendency to radiate from one central stem, somewhat similar to the branches 



1 Mr. S. P. Woodward describes the disk and loop as follows : — Oral processes united, forming a bridge 

 over the small and deep visceral cavity ; disk grooved for the reception of the loop, the grooves separated by 

 branches from a central septum ; loop often unsymmetrical, lobed, and united more or less intimately with 

 the sides of the grooves." ('Manual of the Mollusca,' part ii, p. 221, 1854.) M. Suess's views regarding the 

 loop differ from those we advanced in 1852; but a further examination of the animal of the recent 

 Th. Mediterraneum will be desirable, before this question can be finally settled. The cirrated margin being 

 apparently attached to the inner sides of the sinuous grooves, in the specimen examined by Mr. Woodward 

 and myself, in 1852, might perhaps favour M. Suess's opinion? 



2 I much regret not having been able to reproduce some of M. Suess's admirable illustrations, but I 

 found that they could be but imperfectly represented by woodcuts. 



