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Family DIPLOGRAPTIDAE, Lapworth. 



1873. Diplograptidse, Lap worth, Geol. Mag., vol. x, table i, p. 555. 



Biserial Graptoloidea, with straight stipes. 



Thecal tubular, usually in contact for a considerable fraction of their length ; 

 ventral wall approximately straight or with every degree of sigmoid curvature; 

 apertural margins horizontal or inclined, everted or introverted, occasionally intro- 

 torted. Test continuous, membranous. 



The mode of development of the polypary in the Diplograptidae is in the main 

 similar to that which obtains in the biserial portion of Dicranograptus, and may be 

 described as follows : 



The first theca (th. I 1 ) buds directly from the sicula and on the same side of it 

 as the virgella; it grows steadily in a downward direction until it has first reached 

 the level of the aperture of the sicula, when it bends round, and grows upward and 

 outward. Before making this change in the direction of growth — indeed, probably 

 soon after it has left the sicula — it gives origin to the second theca (th. I 2 ) which 

 crosses the sicula and grows in such a manner that its aperture is on the opposite 

 side of the sicula with respect to that of the first. This second theca (th. 1~) in 

 turn gives rise to a third (th. 2 1 ) whose aperture is vertically above that of the 

 first, and the third in its turn gives origin to a fourth (th. 2~) whose aperture is 

 situated vertically above that of the second. The thecas lying on the same side 

 of the polypary as th. I 1 constitute the primary series (th. I 1 , 2', 3', etc.), those on 

 the same side as the th. I 3 constitute the secondary series (th. I 3 , 2°, 3 2 , etc.). 



In some Diplograptidae this alternate mode of development may persist as long- 

 as the polypary continues to grow r ; and in these forms, although the upper portion 

 of each theca is individualised, the lower portions of all the thecse communicate 

 freely with each other. In many forms, hoAvever, the polypary is known 

 to be divided partially or entirely by a longitudinal septum; in such eases the 

 third theca (th. 2 1 ), (or more exceptionally a later one,) gives rise to two thecos, 

 one (th. 2 2 ) on the same side as the second (th. l'~), and one (th. o 1 ) on its 

 own side, and these two are separated from each other by a septal wall. 

 This septal wall may extend completely through the polypary from the obverse 

 to the reverse side even from the earliest stages ; the septum is then said to be 

 complete (CI. scalaris.). In other cases, however, it appears only on one side 

 (the obverse), and is not visible on the reverse until a later stage in the growth of 

 the polypary; the septum is then said to be incomplete {('I. medius, CI. Tornquisti). 

 Every theca developed after the commencement of the septum buds from the 

 theca next below it belonging to its own series. 



As in Dicranograptus, the crossing canal, as such, in the Diplograptidae is so far 

 reduced as to be practically non-existent. 



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