293 
Campanulinidae shows that we have to discern between 6 different 
types, and in two of these the operculum is forraed of the whole 
hvdrothecal roof. This is the case with the univalvular adcauline 
operculum of Abietinella, and with the three- or four-valvular oper¬ 
culum in Thyroscyphus, and Tetropoma. 1 ) A third type is repre¬ 
sented by the operculum of Calycella , which, as shown by Krarup, 2 ) 
is formed of the peripheral part of the roof, while in the three 
last types it is formed of a smaller or larger distal part of the 
side-wall after the roof has been thrown off. Tn Toichopoma it is 
formed by an infolding of the side-wall at the one side, while in 
Cusptdella, Lafoeina, Oplorhiza, Campanulina and Opercularella 
it is formed of the whole distal part of the side-wall. The sixth 
type is lepiesented by the operculum of Stegopoma about which 
we have alieady spoten. The difference between the named forms 
of opeicula is leally so great, that there can scarcely be any doubt 
but that they have developed independentlv, and I s hall here 
point out the significant phenomenon that a number 
of di f fe i en t forms indopendently and in different 
man ner have developed a protecting roof to the hydro- 
t h e c a. 
I have already given my reasons why I must regard the pre* 
sence of an operculum as a family character, and I shall lastly 
add that if we were to refer all these operculate forms to the 
Lafoeidae we should also be obliged to refer the Sertulariidae to 
the latter family. 
A comparison between the diagnoses given above of the two 
opeiculate families, the Campanulinidae and the Sertulariidae, shows 
that they are very nearly related, and the most significant charac- 
teis, which distinguish the members of the latter family, are that 
the^ are always bilaterally developed, always sessile, and as a rule 
have a larger or smaller part of the adcauline wall coalesced with 
the corresponding stem or branch. Further in all the Sertulariidae 
This genus must, no doubt, be united with Thyroscyphus. 
2 ) 30, p. 380. 
