272 D. M. 8. Watson— The Loop of Dielasma. 



Y, — The Loop of Dielas.va. 



By D. M. S. Watson, B.Sc, Beyer Fellow in the Geological Department of the 

 Victoria University of Manchester. 



ONE of the most important investigations carried through by the 

 late C. E. Beecher was concerned with the development of 

 the loop in an American species of Dielasma, JD. turgidum, from the 

 St. Louis group of the Lower Carboniferous of Kentucky. 



Beecher showed that the loop in early stages was similar to that 

 observed in the adult individuals of the genus Centronella, consisting of 

 two simple short brachial processes united at their expanded anterior 

 ends. I'his development is of much importance from a phylogenetic 

 standpoint, suggesting as it does the development of the Tere- 

 bratulids and Helicopegmata from forms like the Rhynchonellids and 

 Pentamerids. 



So far as I know, it has not yet been confirmed by observations 

 on other species, so that I am glad to be able to describe some 

 similar stages in Dielasma hastata from the uppermost Carboniferous 

 Limestone of Castleton, Derbyshire, and D. elongata from the Middle 

 Zechstein of Posneck, in Thuringia. These last specimens belong 

 to the Manchester Museum, and were collected and determined by 

 Geinitz. The loops in these specimens were developed by the method 

 invented by the Rev. N. Glass, which I found to be easily applied 

 even to quite small shells. 



D. HASTATA (Sow.). 



An example 4 mm. long shows a very small typically centronellid 

 loop, about I'omm. long and under 1mm. wide. The anterior end 

 of the brachial processes are not very much expanded in this specimen. 

 A specimen 10 mm. long shows a loop remarkable for the extra- 

 ordinary resemblance it presents to the adult Centronella loop. In 

 this case the anterior ends of the brachial processes are expanded, 

 and unite to form a strong ridge on the ventral side, which ends 

 both anteriorly and posteriorly in a point. The jugal processes 

 are quite distinct. The whole arrangement of the loop in this 

 example will be easily understood from Pigs. 2 and 3. An individual, 

 15 mm. long and 12 mm. wide, shows a typical Dielasma loop with 

 ascending and descending branches. 



D. ELONGATA (v. Sch.). 



An example of this species 6 mm. long by 5 mm. wide shows 

 a loop very closely resembling that of the D. hastata 10 mm. long. 

 It is drawn as Pigs. 6 and 7. Three specimens of D. sacculus vel 

 vesiculosa under 10mm. in length showed the adult loop. 



Although these observations are very incomplete, it being not easy 

 to get small shells in suitable condition for development, I think they 

 are worth publishing as affording corroborative evidence of the type 

 of development found in the American species. 



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