304 BULLETIN 77, UNITED STATES NATIONAL MUSEUM. 



DITTOPORA ANNTJLATA (Eichwald). 

 Plate 3, fig. 5; plate 11, figs. 16, 17; text figs. 186, 187. 



Chaetetes annulatus Eichwald, Lethaea Rossica, vol. 1, sect. 1, 1860, p. 480, 



pi. 28, figs. 2a-2c, 3a-3d. 

 DiWopora anntiZaia Dybowski, Die Chaetetiden derOstbaltischen Silur-Formation, 



1877, p. 86, pi. 2, fig. 5.— Nicholson, The Genus Monticulipora, 1881, p. 234, 



fig. 49. 



Zoarium of small, ramose, frequently dividing branches 3 to 6 mm. 

 in diameter. Surface finely spinulose on account of the numerous 

 small acanthopores; maculas of thick- walled mesopores usually 

 closed at the surface, and encirchng the branch, giving the character- 

 istic annulated appearance. In the early portion of the mature 

 region the usual pair of larger acanthopores, one on each side of a 



Fig. 186.— DiTTOPOEA ANITOLATA. copy of ElCffWALD'S ILLTTSTEATIONS OF CiLETETES ANNT7LATU3. B, 

 FRAGMENT OF A NORMAL MATURE ZOARIUM, NATURAL SIZE; 6, SMALL PORTION OF THE SAME MAGNIFIED 

 TO SHOW THE ARRANGEMENT OF THE RING-LIKE MACULE AND THE DISTRIBUTION OF ZOCECIA AND MESO- 

 PORES; C, END VIEW OF A FRAGMENT ENLARGED; d, VIEW OF CELLS IN SECTION; € AND/, FRAGMENT OP 

 AN OLD EXAMPLE AND A PORTION ENLARGED. HERE THE ZOCECIA ARE ALMOST ENTIRELY OBLITERATED 

 BY THE EXTENSIVE DEVELOPMENT OF MESOPORE-LIKE CELLS; Q, END VIEW OF BRANCH, MAGNIFIED. 

 "CaLCAIRE 1 OETHOCERATITES," PULKOWA and PoPOWKA, GOVERNMENT OF ST. PETEESBUEG. 



zooecium, is present, but numerous smaller acanthopores develop in 

 the later portions of the same zone and become so nearly equal in 

 size that the distinction between the two sets is lost. The tangential 

 sections shown in figure 187 a and & are through this older portion 

 of the mature region. Dybowski's figure (pi. 3, fig. 5) represents 

 the larger set more distinctly, and also shows the hour-glass shape 

 imparted to the zocBcia by the indentations of the walls whenever a 

 pair is well developed. Vertical sections show essentially the same 

 structure as in D. clavseformis. Semidiaphragms are fewer in D. 

 annulata and occur mainly in the bend to the mature zone. 



While it is recognized that D. clavseformis and D. annulata are 

 closely related, the ramose habit of growth of the latter is the most 

 obvious difference. The ring-like maculae are not of specific impor- 

 tance, because they are equally well developed in all of the other 

 species of Dittopora. 



