200 



BULLETIN 77, UNITED STATES NATIONAL MUSEUM. 



MESOTRYPA EXPRESSA, new species 

 Text fig. 108. 



This ratlier unusual form is particularly interesttug in showing to 

 how great an extent mesopores which a,re abundant in the less mature 

 portions of a colony may be pinched out in the more mature zones. 

 This feature is best seen in vertical sections (fig. 108 e), although 



Fig. 108.— Mesotkypa espeessa. a, side veew of the type-specimen, gkowing AEOxnsiD A Hyolithes; 



6, VIEW OF THE SAME EXAMPLE FRACTURED LENGTHWISE; C, TANGENTIAL SECTION, X20, THR0X7GH THE 

 MATURE REGION, ILLUSTRATING THE SCARCITY OF MESOPORES; d, A PORTION OF THE SAME, X35, "WITH 

 THE -WALL STRUCTURE MORE CLEARLY INDICATED; fi AND/, TWO VERTICAL SECTIONS, X20, DIFFERING 

 ONLY IN THE DEVELOPMENT OF MESOPORES IN e, WHERE THEY DISAPPEAR BEFORE THE SURFACE IS 

 REACHED. KUCKERS SHALE (02), REVAL, ESTHONIA. 



tangential sections prepared from different parts of the zoarium will 

 exliibit the same variation in the number of mesopores. Such a 

 pinching out or disappearance of the mesopores as growth proceeds 

 is a characteristic of the genus as a whole, although it is seldom so 

 well shown as in the present instance. 



The three zoaria studied form small, elongate masses incrusting a 

 species of Hyolithes. The fractured specimen from which the figured 

 thin sections were prepared is shown in figure 108 h, where the layers 

 of zooecia are seen proceeding from the clay filled body of the shell. 



