20 DE. T. DAVIDSON ON BECEXT BEACHIOPODA. 



Iielonging to the ribs are antler-shaped, that is, twice, thrice, or oftener subdivided. 

 Occasionally the perforations are confined to the ribs, which causes 

 them to lie in bauds. It is only by the most eai'eful examination, 

 while grinding down the ribs to the level of tlie intermediate furrows, 

 tliat the true forms of the perforations can be determined. As the 

 modification in question is a very important one in many respects, 

 I have given two representations of it taken from a section prepared 

 l)y Dr. Rowney. Pig. 10 shows a band of antler-shaped joerforations 

 in the simplest state, magnified 60 diameters : it would have been 

 difficult to have represented them under the complex form they some- 

 times assume. Eig. 11 shows a portion of one of the perforations, magnified 210 diameters, 

 having each termination of its branches furnished with a brush-like bundle, which in 

 tliis species, as in Terehratula vitrea, is smaller than usual." [See woodcut, fig. 1 A & B.] 



The shell-structure of T. ca2)ut-serpentis being so very remarkable, I asked my friend 

 Mr. John Young, of the Hunterian Museum, Glasgow, to carefully examine the shell- 

 structure of this species. He writes me on the 20th of April, 1885 : — " The shell-structure 

 of this species is of great beauty. The numerous canals incline from the lower margin 

 of the shell upwards towards the beak, but on the ribs of the shell they radiate from 

 llie centre of each rib to either side, there being a central row of tubes or canals that 

 ])oint neai'ly straight upwards, while the other rows diverge. It is this bending inwards 

 of the tubes on each side of the ribs towards the interior of the shell that caused Dr. 

 Carpenter, in the introduction to your monograph, to state that they open internally in 

 rows. Dr. Carpenter is, however, wrong when he states that the pores or canals are 

 principally seen on the elevated parts of the plications (in this sjiecies), my etched speci- 

 mens show that they ojien quite as numerously in the hollows between the ribs. It is 

 only internally that the perforations gather together into rows with bare spaces between. 

 On the exterior surface of the valves the pores are more evenly distributed, owing to 

 the divergence of the tubes on the ribs. The perforations are not so well seen on this 

 species, previous to etching. The tubes on their passage through the shell-surface often 

 have a wave outward and upwards." 



The embryology of Terehratulina caput-serpentis and of T. septentrionalis has been ad- 

 mirably investigated by Prof. E. Morse * and Prof. Kowalevsky t- Prof. E. Morse says : — 

 " In the first stage the embryo becomes widened at one end. The segments are barely indi- 

 cated, tlie posterior end is the widest, the anterior portion is ornamented with a conspicuous 

 tuft of long cilia, so peculiar to the embryos of many worms. The embryo is also clothed 

 with vibratile cilia, and in this condition slowly moves along the bottom of the dish 

 \vithout rising from it, or remains quiet. In the second well-marked stage the embryo 

 is divided into two prominent segments ; these expand and contract upon each other 

 slightly, and the cephalic segment has the power of partially bending from side to side. 

 In tills stage tlie embryo is most active, swimming rapidly in every direction and turning 



* "Embryology of TirchratiiUna," Mem. Boston Soc. Nat. Hist. vol. ii. pp. 251-264, 1873; and "The 

 SyBtematic Position of the Lrachiopoda," Proc. Boston >Soe. Ivat. Hist. vol. xv. pp. 315-372, 1873. 

 t ' Untersuchungeu iibcr die Embryologie der Brachiopodeu :" Moskau, 1874. 



