168 On the Genus Tetradium, Dana. 



positive evidence is of much greater weight than the apparent 

 want of septa in the other sections, we feel satisfied that we 

 are right in refemng our specimens to Tetradium, of which 

 thej constitute the only recorded British species, and the most 

 delicate of the hitherto described species of the genus. The 

 number of septa seems to be generally from three to five ; but 

 it is very difficult to determine this point satisfactorily, as the 

 gi'anular walls of the minute tubes become almost undistin- 

 guishable from the infiltrated calcite, if a higher magnifying- 

 power than a one-inch objective be employed. 



In longitudinal sections the corallites are seen to have the 

 form of delicate wavy tubes (figs,/, ^), running parallel to 

 one another, and often arranged in a succession of superposed 

 layers. The corallites are crossed by numerous complete 

 tabulaj, placed about the diameter of the tubes apart. As the 

 tabulae appear to be very generally placed at the same level in 

 contiguous tubes, the longitudinal section looks under the 

 microscope like a piece of finely- woven cloth (fig./), and 

 it is sometimes difficult to determine which of the two sets of 

 intersecting lines are the walls of the corallites and which are 

 the tabulae. Whether the coraUites are amalgamated by their 

 walls or not, and what is their mode of increase, are points 

 which we have been unable to decide. The diameter of the 

 corallites is not absolutely uniform ; but appears to be gene- 

 rally between one three-hundredth and one four-hundredth of 

 an inch. Both in longitudinal and in transverse sections, but 

 especially in the latter, are observed certain peculiar canals 

 (fig. fZ), which have a diameter of about a fiftieth of an 

 inch and are placed at variable intervals. They do not ap- 

 pear to take any very uniform course, though generally more 

 or less parallel with the corallites, which are sometimes con- 

 centi'ically disposed around them ; and they are so constant in 

 their occurrence that they must unquestionably belong to the 

 coral. In a small section six lines long and three lines wide, 

 nine of these canals are cut across ; but they are seldom as 

 close-set as this. They strongly call to mind certain canals 

 which are to be observed in many of the Strom atoporoids ; and 

 their function can hardly be other than that of conveying 

 water from the exterior to all parts of the dense corallum. 

 They would appear, therefore, to have a merely physiological 

 value. 



Specimens of Tetradium PeacMi were first brought under 

 the notice of one of us (R. E., jun.) by Messrs. Henderson and 

 Brown (Edinb. Geol. Soc), who collected examples at the 

 Habbies Howe. The organic nature of the spheroidal masses 

 was first detected by our friend Mr. C. W. Peach, A.L.S., 



