DE. T. DAVIDSON ON EECENT BEACHIOPODA. 69 



Plate IV. 



Figs. 1-5. Various stages of growth of Terebratidina caput-serpentis, Liniie, sp. 1, 2. Large specimens 

 in the Zool. Dcpt., British Museum. 3, 4, 4«, 4 b. Specimens of ordinary size from the coast 

 of Scotland. 5. A very young specimen (after E. Deslongchamps). 



Fig. 6. A young specimen in which the crura of the loop are not yet united. 



Fig. 7. Interior of the dorsal valve of TerebratuUna caput-serpentis (after E. Deslongchamps), showing 

 the labial appendages and the cirri, ac, cardinal process ; vc, visceral cavity ; b, mouth ; be, 

 descending or primary portions of the arms ; br, ascending or reflected portions of the arms ; 

 mi, interhranchial membrane ; e, shield ; sp, median poi'tion of the arms spirally coiled. 



Fig. 8. Dorsal view of Terebratidina caput-serpentis deprived of its shell (after Hancock), n, pallial 

 lobe ; b, body ; c, peduncle ; d, d, great pallial or genital sinuses ; e, ramifications of ditto ; 

 /, muscular ties passing between the walls of ditto ; ij, genitalia seen through the walls of 

 ditto ; h, marginal fold ; i, setae ; j, extreme pallial margin ; k, depressions corresponding to 

 the bases of the crura ; /, ridge formed by the union of the pallial margins ; m, edge of dorsal 

 mesentery; n, liver seen through the walls of the perivisceral chamber; o, extremities of 

 anterior occlusors ; p, ditto of posterior occlusors ; q, ditto of divaricators ; r, ditto of dorsal 

 adjusters. 



Fig. 9. Ventral view of the same (after Hancock) ; the letters up to j agree with those of fig. 8. k, ex- 

 tremities of occlusor muscles ; /, ditto of divaricators ; m, ditto of ventral adjusters j n, ditto 

 of peduncular muscle ; o, peduncular nerves. 



Fig. 10. Interior of the ventral valve of T. caput-seipentis (after E. Deslongchamps), showing the mantle 

 in its entirety, vc, visceral cavity ; ap, raised portion of the mantle forming the wall of 

 the visceral cavity ; o, organs of reproduction ; svl, lateral venous siniises ; svl*, trunks 

 accruing from the division of the sinuses ; svm, median venoiis sinus ; r, small branches of the 

 venous sinuses ; on all the circumference one sees distinctly the festooned muscular border of 

 the mantle, whereon arc implanted a multitude of hairs (enlarged 5 diameters) . 



Fig. 11. Interior of the dorsal valve of the same species (after E. Deslongchamps), showing the mantle 

 also in its entirety. The same letters indicate the same jiarts on this valve, and the parts cor- 

 respond with those of the ventral valve, with the excejition of ab, which shows the brachial 

 apparatus in the shape of a ring (enlarged 5 diametez's). 



Plate V. 



Figs. 1-30. TerebratuUna septentrionalis, Couthouy, after Morse, "Embryology of TerebratuUna," Mem. 

 Soc. Boston Nat. Hist. vol. ii. pi. viii., 1873. — 1. Acluster of eggs from the genital band. 2,3,4. 

 Eggs from the perivisceral cavity immediately after their escape from the pallial sinuses. 5. Firet 

 ciliated stage. G, 7 , 8. Successive stages of transverse division of embryo, showing long tuft of 

 cilia at cephalic extremity. 9, 10, 11. From a single embryo, showing various outlines assumed 

 while swimming. 12, 13, 14. Different embryos showing first appearance of caudal or pedun- 

 cular segment. 15, 16. Representing the same embryo contracted and expanded. 17, 18, 19. 

 Embryos just attaching themselves by their caudal segment. 20. Embiyo showing first appear- 

 ance of ventral (?) area by the bulging of the thoracic ring. 21, 22. Embryos in various 

 stages showing widening of thoracic ring, and its gradual growth towards enclosing the 

 cephalic ring. 23, 24, 25. Successive stages of the embryo, showing formation of dorsal and 

 ventral areas by the folding and growth of the thoracic ring. In fig. 26 the head is still seen 

 projecting from the dorsal and ventral folds of the thoracic ring. 27 to 30. Succeeding stages 

 of embryos drawn in various positions. The deciduous setae appear in these stages. 



Fig. 31. One of the earliest stages of TerebratuUna septentrionalis, after Morse {op. cit. pi. i. fig. 3), 

 " in which the body has rapidly lengthened, and the peduncle is equal in length to the 



