107 



these appendages for the purpose of classification it is necessary 

 to subdivide them into three classes : 1 . the pedunculate ; 2. 

 the sessile ; 3. the immersed, the two latter classes, however, 

 run insensibly into each other, whilst the pedunculate form is 

 obviously quite distinct, inasmuch as it presents an additional 

 member in the form of a basal joint. It is to this form of avicu- 

 larium that the term " bird's " or " vulture's heads " is more 

 properly applied. It occurs in Bugula avicularia, B. plumosa, 

 B.flabellata, B. dentata, and in Bicellaria ciliata; whilst it is 

 wanting altogether in Bugula neritina, Bicellaria grandis and.B. 

 gracilis, species therefore which it is very desirable should be 

 examined in the living or fresh state, for the purpose of deter- 

 mining whether the inhabitant of the cell does not afford cha- 

 racters sufficient to cause these species to be referred to genera 

 distinct from those to which, from consideration of their ske- 

 letons alone, they are now regarded as belonging. A modifica- 

 tion of pedunculate avicularium, where it assumes the form of a 

 long trumpet-shaped or infundibuliform organ, exists in Bicellaria 

 tuba. With the exception of this latter form, the pedunculate 

 avicularia are always placed on the anterior aspect of the cell, 

 on one side below the level of the aperture. 



The sessile form of avicularium, distinguished from the im- 

 mersed) is pretty extensively met with, though it does not occur 

 so abundantly as the latter form. With a single exception 

 (Amastigia nuda, PL XXX VI.), this form of avicularium is almost 

 invariably placed upon the front of the cell below the aperture, 

 as in Alysidium Lafontii (PI. XIV.), most species of the genera 

 Menipea, Scrupocellaria and Canda, and in many of the genus 

 Caber ea ; in some, as in Scrupocellaria ferox (PI. XX11.), at- 

 taining gigantic dimensions. It is distinguished from the im- 

 mersed form of avicularia, not only by its greater projection, but 

 also, and perhaps more essentially, by its being composed of a 

 more or less flexible material ; whilst in the latter form, even 

 where it is somewhat prominent, the cup is always calcareous and 

 rigid. 



The immersed form of avicularium occurs in a great number 

 of genera and species, and in many instances, as in the genus 

 Lepralia, affords excellent specific characters. The variety of 

 appearances and position assumed by this form of avicularium 

 will be best appreciated upon inspection of the figures, especially 

 of the genera Catenicella, Scrupocellaria , Menipea, Caberea, 

 Lepralia, Cellepora, Eschara, Retepora, Emma, Salicornaria 

 and Flustra. In Diachoris crotali, Notamia bursaria and Bicel- 

 laria tuba, the avicularium presents such an aberrant conforma- 

 tion, that it can scarcely be arranged in either of the above 

 categories. That of Diachoris crotali and Bicellaria tuba, how- 



