NIAGARA GROUP. 



Ill 



FAMILY CYATHOPHYLLIDEA. 



505. 7. STREPTELASMA CALIGULA 



Pl. XXXII. Fig. la -k. 



Turbinate, oblique or curved, more or less rapidly expanding by the addition of interstitial 

 rays ; cup moderately deep ; rays or vertical lamellae about half the thickness of the space 

 between them, from 20 to 50, ordinarily about half this number, more or less curved towards 

 the centre ; external surface with the lamellae very distinct and marked by transverse striaa ; 

 surface rarely corrugated ; rays alternating with short dentations on the inner margin of the 

 cup. 



This species, which is very abundant in the shale of the Niagara group, difiers but little in 

 its external characters from some of the species in a higher position. A more careful com- 

 parison, however, shows it to be clearly distinct. From the small Trenton limestone species it 

 differs in having a more shallow cup and stronger lamelte. 



This species presents a striking feature in having the rays mostly increasing on two sides, 

 leaving two central or anterior rays continuous from the base : the increase, if taking place in 

 other parts, does so by interstitial rays on one side only of a ray continuous from the base. 



Fig. 1 a. Anterior view, showing the interstitial or additional rays on each side of two continuous 

 rays. 



Fig. 1 b. Lateral view, showing interstitial rays on one side of a continuous ray. 



Fig. 1 c. The opposite side of the same individual, showing the same character as fig. 1 b. 



Fig. 1 d. View showing the interior of the cup, and posterior or shorter side of the coral. 



It will be seen that there is no increase on this side by interstitial rays, and consequently the 

 cup does not increase so fast as on the other side, which is not only expanded by this 

 addition of new rays, but is built upwards more rapidly, producing the form which 

 we see prevailing not only in this but in many other species. This increase of the 

 Cyathophyllidea on one side always takes place equally on each side of the larger or 

 anterior ray ; though in many specimens, and perhaps in all of the Genus Strepte- 

 LASMA, there are no means of distinguishing the larger ray except from the mode of 

 increase. The feature presented in fig. 1 a is, however, of equal interest and im- 

 portance, and corresponds to the larger ray in other examples. 



Fig. 1 e. Transverse section, showing the number of rays with the alternating dentate processes. 



Fig. I f. A vertical section, showing the depth of the cup. The margin above is broken off. 



Fig. 1 g. An unusually large individual of this species : lateral view. 



Fig. 1 A.. An individual more straight than usual. 



Fig. I i. A small individual with the enveloping membrane or cup entirely destroyed, leaving 

 only the rays« 



