86 PROF, GROOM ON POLYPHYMA LAPWORTHI. [F eb. 1902, 
it is therefore to be presumed that, unless this is due to pressure, 
the valves when closed did not gape at any point along the ventral 
margin. No differences between different individuals whi¢h could 
be attributed to sex were observed. 
The substance of the valves is thin, black, and shining, and evi- 
dently consisted originally of chitinous material. 
The length of the valves generally varies from 1 to 3 millimetres, 
and the height from 0:5 to 18mm. The most abundant indi- 
viduals measured a little under 2 mm. by a little over 1 mm. 
Measurements of a number of selected individuals gave the fol- 
lowing dimensions (in millimetres) :—1l1:0x0°5; 1:24x0-66 ; 
16x 0°8; 1°75 *1:0; 19x11; 20x11; 2:26 x 1:32 ete alae 
2-6 x 1:56; 2°83 x 1-ba;and'2-9 x 1°8 (the biggest complete individual 
seen). A few specimens showed indications of greater size; the 
largest of these, an impertect specimen measuring oO. 6 mm. in height, 
must have been some 4 or +°5 mm. in length. The average lengths 
and heights of sixteen specimens less than 2 mm. in length were 
1:6 and 0-9 mm. respectively ; the corresponding measurements of 
ten larger individuals were 95 5and1:4mm. The relative height 
thus appears to increase with age. 
III, Retarion to ALLIED GENERA. 
The genus Beyrichia was instituted by McCoy.’ In subsequent 
years many new species were referred to this genus by different 
observers. Latterly, however, certain of these have been separated 
off to form distinct genera, and other genera having been added, the 
Leperditiadee now include a number of lobulate forms. Among 
the lobulate genera recognized are the following :—Beyrichia,’ 
Ctencbolbina,’ Tetradella,' Bollia,’ Strepula,® Polyzygia,’ Poloniella,® 
Jonesella, and Drepanella.° The mutual relations between many 
of these forms is very obscure, and it appears doubtful whether 
all of them are entitled to rank as genera. The genera to which 
Polyphyma appears to be most nearly related are those pro- 
vided with broad lobes. such as Aladenia, Beyrichia, Ctenobolbina, 
and Zetradella. The arrangement of the lobes, however, is more 
complex than that seen in Aladeiia or in the simpler forms of 
Beyrichia aud Ctenobolbina ; moreover, it does not seem possible to 
regard Polyphyma as hee ce originated, like the more complex forms 
of Beyrichia, from the simple three- lobed type. On the other hand, 
1 «Synops. Silur. Foss. Irel.’ 1846, p. 57. 
> See T. Rupert Jones, Ann. & Mag. Nat. Hist. ser. 2, vol. xvi (1855) pp. 81, 
163; & G. Reuter, Zeitschr. Deutsch. Geol. Gesellsch. vol. xxxvii (1885) p. 621. 
3 FB. O. Ulrich, Tanne Cincinnati Soc. Nat. Hist. vol. xiii (1891) p. 108, 
* Ford, p. 112. 
> T. Rupert Jones & H. B. Holl, Ann. & Mag. Nat. Hist. cer. 5, vol. xvii 
(1886) p. 360. ° Ibid. p. 403. 
7G. ‘Giirich, Verh. Russ. Kaiserl. Mineralog. Gesellsch. St. Petersb. vol. xxxii 
el 18§ 396) p. 387. * Ibid. p. 388. 
' FE. O. Ulrich, Journ. Cincinnati Soc. Nat. Hist. vol. xiii (1891) p. 121. 
10 Fbid.op. UT. 
