13G Filing: EcoR1 Capital and Marinus Pharmaceuticals Inc (MRNS)

UNITED STATES

SECURITIES AND EXCHANGE COMMISSION

Washington, D.C. 20549

SCHEDULE 13G

Under the Securities Exchange Act
of 1934
(Amendment No. _)

Marinus Pharmaceuticals, Inc.

(Name of Issuer)

common stock, par value $0.001 per share

(Title of Class of Securities)

56854Q101

(CUSIP Number)

August 4, 2017

(Date of Event Which Requires Filing
of this Statement)

Check the appropriate box to designate the rule pursuant
to which this Schedule is filed:

[X]       Rule
13d-1(b)

[X]       Rule
13d-1(c)

[ ]       Rule
13d-1(d)

* The remainder of this cover page shall be filled out for
a reporting person’s initial filing on this form with respect to the subject class of securities, and for any subsequent
amendment containing information which would alter the disclosures provided in a prior cover page.

The information required in the remainder of this cover page
shall not be deemed to be “filed” for the purpose of Section 18 of the Securities Exchange Act of 1934 (“Act”)
or otherwise subject to the liabilities of that section of the Act but shall be subject to all other provisions of the Act (however,
see the Notes).

Potential persons who are to respond to the collection
of information contained in this form are not required to respond unless the form displays a currently valid OMB control number.