58 INTRODUCTION. 



Prof. King and M. Suess it seems evident that the arms were largely supported by a remark- 

 able process somewhat in the shape of a loop, but from the inner edge of which numerous 

 lamellcB branched off. The place this genus should occupy in the classification must 

 therefore remain unsettled until a complete knowledge of the calcified process has been 

 attained. 



3. In the Thecideid^ the oral arms have not been as yet perhaps completely under- 

 stood, and it will be very desirable to examine fresh examples of the animal of the only 

 species we now find alive. In the greater number of fossil forms there exists a more or 

 less complicated disk variably grooved for the reception of a peculiar loop or apophysary 

 ridge which seems to become free near the visceral cavity; but other important peculiarities 

 in the general habit of the genus Thecidium has necessitated its separation from the 

 Terehratulidce, to which it appears allied in many respects. Important discoveries recently 

 made by M. Suess, of Vienna, may cast additional hght on this group, which is so par- 

 ticularly worthy of the attention of the Palaeontologist. 



4. The SpiRiFERiDiE may be easily and clearly characterised by the largely-deve- 

 loped spiral lamellae which form two horizontal or vertical cones filling the greater portion 

 of the shell, and no doubt supported spiral cirrated arms : but we may likewise infer 

 from the extent of these processes that the oral appendages were not extensile, as may pos- 

 sibly have been the case in Bhpichonella. Two principal modifications in the position 

 and direction of the spires have been ascertained, that common to Spirifer, and the other 

 to Atrypa of Dalman {reticularis, marginalis, prumtni, &c.,) various other details of internal 

 arrangement and shell-structure have rendered desirable further subdivisions of the group, 

 but in all the character of the spire prevails. Some of the species, as we have already 

 noticed, bear a close outward resemblance to certain Terehratulidce, but active researches 

 have hitherto brought to light but rare instances of true Terebratulce with loops in the 

 Palaeozoic epoch, and on the contrary the generality of these ancient terebratuliform shells 

 were provided with calcified spires. 



5. The only other genus hitherto discovered which possessed spiral processes for the 

 support of the labial appendages, not here included among the Spiriferidce, is that pro- 

 posed by M. Suess under the name of Koninckina (PI. VIII, figs. 194 — 198), for a 

 shell confounded with Productus, but which the Viennese author found to possess spiral 

 lamellae somewhat similar to that of Atrypa, but we have not ventured to include this 

 genus for the present in the Spiriferidce, although its place cannot be far removed. 



6. In the great family RnYNCHONELLiDiE, the labial appendages seem to have been 

 free, spiral, fleshy, perhaps extensile, and supported only at their origin by two short calci- 

 fied curved lamellae. The family includes the genera, viz. Ithynchonella, Camarophoria, and 

 Pentamerus. 



7. But little is known of the labial appendages in the families STROPHOMENiD.a; and 

 Productid^ ; and in the sub-families Poramobnitidce and BavidsonidcB, the arms were in 

 all probabihty fleshy and spirally rolled, evidence of which may be observed in the interior 



