AND SPECIES OP HYDROIDA. 283 



Plate XVI. 



Thuiaria crassicaidis. 



1. A nearly perfect colony, natural size. 



2. Portion of a bifurcating branch, magnified. 



3. Portion of same, more magnified ; lateral view of hydrothecu; 



with gonangium. 



4. Same, front view of hydrothecse. 



5. Transverse section of stem near proximal end, magnified, show- 



ing the great central irregular canal and the peripheral canals 

 in the thick chitinous perisarc. 



Plate XVII. 



1-3. Thuiaria coronifera. 



1. Natural size. 



2. Part of a branch, magnified. 



3. Hydrotheca, front view. 



4-6. Thidaria persocialis. 



4. Natural size. 



5. A portion, magnified, showing the tendril-like processes. 



6. Hydrotheca, front view. 



Plate XVIII. 



1 , 2. Thuiaria bidens. 



1. Natural size. 



2. A portion, magnified. 



3, 4. Thuiaria ccrastium. 



3. Natural size. 



4. A portion, magnified. 



Plate XIX. 



1, 2. Selaginopsis fusca. 



1. A portion of the hydrophylon in its natural condition, mag- 



nified. 



2. A portion after having been boiled in a solution of caustic pot- 



ash, showing the form and relation of the hydrothecte. 

 (For Selaginopsis ftcsca, natural size, see PI. XII. fig. I a, a.) 



3, 4, 4". Thuiaria doUchocarjm. 



3. Natural size. 



4. Portion of a pinna, magnified, with proximal portion of hydro- 



theca. 

 4°. Distal portion of same hydrotheca. 



