274 The American Geologist. May, ism. 
characteristics of the genus or its being a genus at all. He 
finds that it is simply a form of Petraia. the peculiarity of which 
is stated to be the absence of tabulae and dissepiments. Re-exam- 
ining his sections he is certain that tabulae are present. He next 
finds that it is properly separated as a genus, but that the distin- 
guishing characteristic is the absence of ijjitheca. Can it be that 
he has been mistaken in supposing this to be present ? No, a ref- 
erence to his specimens again convinces him that his former obser- 
vations were correct. Puzzled but persevering he learns next that 
the absence of dissepiments is the one feature by which it must he 
separated from all other genera, He recalls some of the species of 
Amplexus and Zaphrentis. The next reference volume he opens 
he confidently expects to find absence of wall or se/>(a affirmed to 
be the distinguishing characteristic, but the genus is simply made 
a sub-genus of Zaphrentis or referred to Ptychophyllum. One 
very common Silurian coral has been assigned no less than twenty- 
six different names, and others have nearly as long a list of 
aliases. 
This unfortunate condition of affairs seems to have arisen from 
the fact that many descriptions have been based entirely upon ex- 
ternal characters or upon a limited number of poorly preserved 
fossils. Too many genera and species have been founded through 
ig'norance of or ignor'ance of the publications of others. A de- 
gree of satisfaction ma}- be obtained only by collecting the litera- 
ture of each genus and making a comparative study of this in 
connection with the fossils themselves. The value of one's con- 
clusions will then depend upon the number and character of his 
specimens and the accuracy of his observations. 
Another source of embarrassment to the student is the apparent 
blending of the genera in many instances. He will find specimens 
of Cystiphyllum in which the tabula? and septa have reached no 
inconsiderable degree of development, and specimens of Helio- 
phyllum in which the carinas are almost or entirely wanting. 
However, if the commonly accepted theories for the creation of 
genera and species be true, the surprise would be the greater if 
these intermediate forms were not to be found. Let the student 
early learn the meaning of "type classification" and appreciate 
the fact that the limits of genera are drawn by man and not by 
nature. He must also bear in mind that our Palaeozoic corals are 
