456 Mr. J. Thomson and Dr. H. A. Nicholson on the 



compound, but it possesses no representative of the great 

 central cellular mass, formed bj obliquely ascending and vesi- 

 cular tabulas, and traversed bv vertical lamellae, which is so 

 characteristic of the former. On the other hand, a relation- 

 ship of real affinity subsists between CUsiophyllum and Lons- 

 daleitty the great central columella of the latter representing 

 the central vesicular mass of the former, and being in many- 

 respects formed in nearly the same way, though on a much 

 smaller scale. Lonsdaleioj however, is distinguished from 

 CUsiophyllum by being always compound, by increasing by 

 calicular gemmation, and by the fact that the wide and loose 

 vesicular tissue of the outer area is not traversed by the septa, 

 which thus are not in connexion with the wall. 



The genus Cydophyllum, Duncan and Thomson, though 

 in some respects allied to CUsiophyllum^ is distinguished from 

 it by the fact that the bottom of the calice exhibits a secondary 

 circular cup, in place of the conical boss of the latter. On 

 section this cup is seen to be the upper extremity of a great 

 central tube, Avhich passes downwards to near the lower ex- 

 tremity of the visceral chamber. The boundaries of the 

 central tube are formed by a distinct accessory wall ; and its 

 enclosed space is filled with delicate vesicular tissue, becoming 

 more or less irregular and spongy in the centre. The genus 

 Aulophyllum^ Edw, & H., is separated from CUsiophyllum by 

 characters very much the same as those which serve to distin- 

 guish Cydophyllum from the same, especially by the fact 

 that, in common with Cydophyllum, it possesses a secondary 

 cup in the centre of the calice, in place of a conical boss. 



The genus CUsiophyllum, finally, is more or less intimately 

 related to several groups of forms which we shall proceed to 

 describe under the names of Dilmnojphyllum, Aspidophyllum^ 

 Kumatiophyllum , and Txliodophyllum. All these forms may 

 be unhesitatingly regarded as being modifications of a common 

 type ; and it need, therefore, excite no surjjriseto find that the 

 ground-plan of their organization is the same. At the same 

 time the differences which separate tliese several groups from 

 one another and from CUsiophyllum are constant in a large 

 number of individuals in each case, and are easily recognized 

 in typical examples. We cannot, therefore, avoid recognizing 

 the actual existence of these groups as natural assemblages, 

 irrespective of the fact that specimens can be procured which 

 are intermediate in their structural characters between these 

 groups, and thus link them on to one another. As these forms 

 also are separated from one another by characters which can 

 be readily determined in practice, we have judged it better to 

 assign to each group a distinctive name. In so doing, how- 



