50 LOWER PENINSULA. 



Tores well developed. Diaphragms rarely observed excepting as 

 opercLila. Diameter of orifice a little over one millimeter ; internal 

 tube portions half a millimeter. Found in the Helderberg group 

 of Mackinac Island, at Port Colborne, C. W., and likewise in the 

 drift of Ann Arbor. 



Plate XIX. — Fig. 2 is a silicified fragment found in the drift of 

 Ann Arbor. 



CLADOPORA CRYPTODENS, Billings. 



Alveolites cryptodens, Billings. 



Cylindrical polyp stems, from five to ten millimeters in diameter, 

 with distant, dichotomous, straddling ramification. Tubes opening 

 wath oblique dilated orifices, which either join with acute edges, 

 each of them being a circumscribed pit, or have the inner part of 

 the walls spread into an indefined, common interstitialsurface, from 

 which the convex lips forming the front margin of the orifices pro- 

 ject like the teeth of a rasp. Tube size variable in different speci- 

 mens ; in the variety with larger tubes the transverse diameter of 

 the dilated orifices is about one and a half millimeter, the interior 

 cavity of the channels measuring about a half of one millimeter. 

 In the variety with smaller tubes the orifices measure about one 

 millimeter externally, and the internal channels one third of a milli- 

 meter. The tube cavities usually appear to be smooth, but in well- 

 preserved silicified specimens, cleared by acids of the surrounding 

 limestone, the tube channels exhibit three crests, two projecting 

 from the exterior side of the wall, and one intermediate between 

 the two, from the opposite inner wall side. These crests are not 

 noticeable on the dilated orificial part of the tubes, but are distinctly 

 seen in the neck of the channel, where the narrower part begins. 

 Transverse diaphragms are rarely found developed. Pores are 

 large and irregularly dispersed. The crested condition of the 

 tube channels would bring this species under the genus Limaria, 

 or perhaps under Alveolites, w^here Billings placed it, but, consider- 

 ing the general habitus of the specimens, I have placed them under 

 Cladopora, as being nearest related to the forms composing this 

 genus, which is only deprived of crests through the incomplete de- 



