80 THE VOYAGE OF H.M.S. CHALLENGER. 
Genus 9. Symphyllia, Milne-Edwards and Haime. 
Symphyllia, Milne-Edwards and Haime, Cor., ii. p. 369. 
The genera Symphyllia, Isophyllia, and Ulophyllia are accepted as defined by Milne- 
Edwards and Haime. They are closely related to one another, and to Mussa on the one 
hand and Tridacophyllia on the other, but their differences seem sufficiently well marked 
to rank them as distinct. Very different opinions have been expressed by different writers 
as to their relationship, and it is certain that in some cases the forms on which these 
opinions have been based have not been rightfully referred to the genus under which 
they were placed. It was thus a necessary consequence that the genus under which they 
were wrongly placed by a misinterpretation of characters, and the genus to which they 
should have been referred, should not seem distinct, and should therefore have been 
united. 
The numerous species described by Duchassaing and Michelotti from the West Indies, 
and placed by them under the genus Symphyllia, are all forms of Isophyllia, and, as 
Briiggemann has stated, the genus Symphyllia is not found in the West Indies. 
Pourtalés, keeping Mussa distinct, considered that Symphyllia and Isophyllia were 
synonymous, and retained Jsophyllia to include them both. 
Verrill, on the other hand, has maintained that Jsophyllia is distinct from Sym- 
phyllia ; while he has united Symphyllia with Mussa. 
Briiggemann, following Verrill, united Symphyllia with Mussa, maintaining Lsophyllia 
to be distinet from them; he, however, united [sophyllia with Ulophyllia under the 
latter name. 
Duncan, following Pourtalés, regards Mussa and Symphyllia as distinct, but unites 
Symphyllia with Isophyllia under the former name, Ulophyllia remaining distinct. 
It seems to me, however, after a careful study of a large number of species of the 
(fferent genera, that the treatment of these genera given by Milne-Edwards and Haime 
is an accurate one; and they have therefore been all retained, with their original 
signification, 
The essential distinction of Symphyllia from Mussa is to be sought in the nature of 
the wall. In Mussa the walls are normally distinct, a condition which, although most 
clearly seen in those colonies in which the calicles are rapidly isolated, is yet clearly 
evidenced in the development of the seriate forms in which the walls of the developing 
series are found to be free from those of neighbouring series. In Symphyllia, ou the 
other hand, the walls throughout are simple, those of neighbouring series forming a 
simple, solid ridge between the valleys, and originating as such in the earliest stages of 
development. 
The opinion that these two genera should be united, seems to have been based on the 
condition found in such a species as the Mussa vegalis, Dana, in which the walls of the 
