652 BULLETIN 106, UNITED STATES NATIONAL MUSEUM. 



Division INOVICELLATA 



(a) TYPICAL CYCLOSTOMATA 



Family DIASTOPORIDAE Gregory, 1899. 



Cyclostomata tubulnta in which the zooecia are simple, open tubes, which 

 either grow as linear series or as incrusting or erect sheets. The sheets may be 

 coiled into hollow tubes. The zoarium is exceptionally massive. The zooecia may 

 be wholly immersed or partly free. Appendages absent. (After Gregory, 1899.) 



Forma STOMATOPORA Bronn, 1825. 



1825. Stomatopora BHONN, System Urweltlichcn Pflanzenthiere, p. 27, pi. 7, fig. 3. 



1896. GREGORY, Catalogue of the Jurassic Bryozoa in the British Museum, p. 42. 1904. 

 LANG, The Jurassic Forms of the Genera Stomatopora and Proboscina, Geological 

 Magazine, ser. 5, vol. 1, pp. 315-322. 1905. Stomatopora antiqua Haime and its re- 

 lated Liassic forms, Geological Magazine, ser. 5, vol. 2, pp. 258-268. 1907. The evo- 

 lution of Stomatopora clichotomoicles, Geological Magazine, ser. 5, vol. 4, pp. 20-24. 



Zoarium flat, adnate, branching dichotomously, composed of imiserial sub- 

 tubular zooecia. 



Genotype. Stomatopora dichotomy, Lamouroux, 1821. 



Range. Ordovician-Recent. 



The tubes are oval when the peristome is of less width than the width of the 

 tubes. They are cylindrical when the diameter of the peristome and of the tube is 

 practically equal. Again there are some fusiform tubes and others having a club 

 shape. The peristomie is the free part of the tubes forming a prominence above 

 the general zoarial surface ; in the fossil forms it is never very large. The peris- 

 tome is always round ; it remains so when the peristomie is perpendicular to the 

 zooecial plane. Most of the time the peristomie is oblique and by rupture the peris- 

 tome becomes elliptical. 



In our descriptions we never give the size of the aperture. This is a measure- 

 ment which is absolutely inconstant on the same zoarium and the consideration of 

 the diameter of the peristome appears to us sufficient. The determination of 

 species of Stomatopora and of Proboscina is extremely difficult. A knowledge of 

 the ancestrula would probably make the work less difficult, but unfortunately no 

 study of this part of the subject has yet been made. 



The genus Stomatopora has been the subject of much study by Lang, who has 

 established the following points: 



The development of a zoarium is comparable with and follows the same laws as the de- 

 velopment of the zooecium. 



In the genera Stoinatnpnrn and Proboscina the method of branching is of paramount im- 

 portance. 



Two ways of branching may be noticed, namely, lateral branching (ramifica- 

 tion) and dichotomy (dichotomisation). 



In lateral branching a new zooecium arises from any point in a chain of old zooecia, 

 and generally diverges at a wide angle, (see diagram 2, fig. 1, fig. 215R). 



